This answer is impossible.
First of all, the Sages lived under Roman (or Parthian) rule and were in close contact with the gentiles around them, and presumably they knew that those people did not believe and did not accept.
Second, there are Noahide commandments that are disputed in the words of the Sages, so if the Sages themselves, who toiled over their learning, do not necessarily know whether something is forbidden to a Noahide or not, why would some Pliny know?
Necessarily, the Sages simply did not accept the idea of coercion in matters of belief—not that they were unaware of the reality before them, which in this respect is not essentially different from today’s reality. They simply did not interpret reality as coercion. Rather, even if a person toiled and studied and still erred in his analysis, in their view there is still some culpable factor in him, and he is never completely coerced, even if apparently a person has no motive to study.
After all, there is no real difference between some philosopher living in ancient Greece and an atheist nowadays in terms of the level of blameworthiness. If anything, the opposite: a contemporary atheist has a greater chance of changing his beliefs, because knowledge is more available to him and encounters with people of different outlooks are more common.

I completely disagree with you. Clearly, in their view that person is not coerced. But if you ask yourself, you will understand that the truth is that he certainly is coerced. Therefore it is only reasonable to conclude that the Sages really thought he was acting out of inclination and did not recognize the modern phenomenon of the secular person (who does not believe). They attributed it to lack of knowledge or inclination. So he should have learned, and therefore he is negligent and not coerced. As for commandments that are disputed among the Sages, Noahides can clarify what the halakhic ruling is and keep it. And regarding them too, one could say that indeed they would view a Noahide who held otherwise as coerced. The discussion concerns a Noahide who does not observe not because he held a different view.
Bottom line, in my opinion it is impossible to say otherwise.

I completely agree that in their view such a person is not coerced (there is no punishment for coercion). But according to their approach, today’s atheist is also not coerced.
They were aware that gentiles had a developed worldview and beliefs and speculative philosophies (and in that respect this is no different from today’s atheist).
But that was not enough for them to define him as coerced, because they expected even someone very far away and mistaken in his analysis (and who seemingly has no reason at all to act in the way of the Torah) to find the path to the truth, and one who does not find that path is, to some extent, at fault and negligent in their view.
You disagree with them, but you can’t bend their words to your own opinion.
After all, it is as clear as day that an Athenian philosopher or a member of a Teutonic tribe in the Black Forest in the first century CE is closer to coercion than an Israeli atheist today.

If they expected that then I have no argument with them. It’s just that today one cannot expect it. That is exactly what I said. And a gentile in their world was definitely closer than an Israeli atheist nowadays.

Obviously there is an argument.
If someone expected it then, there is no reason he should not expect it today. And how was a gentile closer? What reason would some Greek in Athens have had to think that he needed to believe in the Torah given to a small and remote people hundreds of days’ sailing away, and travel there alone to check whether or not he was allowed to eat the sciatic nerve? And even if he had traveled there, why expect him to believe in the Torah and adopt its commandments? He was not educated in it, and he had a completely different conception of the world.

What was called a philosopher was a negligible and irrelevant minority. The overwhelming majority of gentiles whom the Sages encountered definitely believed in idols and also in the God of Israel, and therefore in their view there is no coercion here.

It is true that the philosophical minority developed over the generations until it became today’s secular majority, but don’t project from the present onto the past.

If anything, here is another question: the Mishnah says that if someone lost a coin and someone else found it, the Holy One, blessed be He, assigns him reward, and apparently according to the Rabbi’s approach there is no reason for that, since there is no commanded act here?
I thought this might fall under “merit is brought about through the meritorious” (within a reality of providence), but it is still difficult.

Most gentiles believed in idols, true. But there is no connection between belief in idols and belief in God and His Torah (and atheists too believe in Spinoza’s God… in both cases it is merely the same name being shared).
And the coercion is the same coercion (perhaps even greater): it too is an error in beliefs and opinions, without any clear motivating factor to investigate more and pursue the hidden truth. Likewise, it does not seem that the Sages regarded the Epicurean and his fellow philosophers as coerced.

Precisely that aggadic statement is not troubling at all. These are words of aggadah, which probably are not speaking about reward for a good deed.

The Sages saw before their eyes people for whom religious belief was obvious, and even if they erred it was only a matter of information and clarification, that’s all. They never dreamed of people who deny because they do not believe—people who, even if everything were explained to them, still would not accept it. The concept of “a child taken captive” reflects this in the best way. From their perspective, it was only a lack of information. Today it is clear to any reasonable person that this is not a lack of information, and therefore applying the concept of “a child taken captive” to our times is problematic.
One could have understood this as simply a mistake of the Sages, and that in fact there were already then clear and deep-rooted deniers like today. But if one accepts their factual diagnosis, the conclusion is as I said: the situation has changed. Back then it was clear to everyone that there is a God and that one must serve Him, and all that was lacking was an honest search and information. Today this is certainly not clear, and information also will not necessarily change the atheist’s position.
Of course there were such figures back then too, but that was a minority that the Sages did not know or at least did not see fit to address because of its insignificance.
It seems to me that we are repeating ourselves.