Maybe he meant earlier times, and today secularism has a certain role? For example, establishing the state and the beginning of the redemption, or sustaining religion in the country.
Even Rabbi Halbertal argued that without secular people, the Haredim would not survive here.
Maybe even to challenge childish faith.

How can it be that secularism is the result of human choices, when it is said: “A person does not so much as stub his finger below unless it is announced about him above”? Does choice constitute a reality separate from Him?

Absolutely. The meaning of free will and freedom of choice is that a person decides, not the Holy One, blessed be He. Those sayings you quote here are either incorrect or must be interpreted differently. Does the Holy One, blessed be He, force a person to desecrate the Sabbath? He explicitly says that He does not want that. I dealt with this in the second book of my trilogy, which is due to come out soon (though I think I’ve also discussed it on the site several times already. You can try searching). If you also want a quote for this from the Sages: “Everything is in the hands of Heaven except the fear of Heaven.” What relates to the fear of Heaven is not entrusted to Heaven but to the human being. One could elaborate, and there are countless sources for this, but it’s unnecessary. It is completely clear from logic and common sense.