The question is whether a story like this could be planted, or whether a tradition passed down from generation to generation, even if it has no historical support from other sources, about a significant event that happened in Shushan and in the kingdom of Ahasuerus, in the presence of masses of people, including large-scale killing, must necessarily be true, similar to the revelation at Mount Sinai.

If not (and it seems from the Rabbi’s words that he is not certain about that here as he is regarding the revelation at Sinai), then what is the difference?

The difference is that with regard to Purim, there is no necessity that the tradition is conveying a real event to us. It could also be a folk story. Greek mythology was also passed down by tradition. The revelation at Sinai—the tradition itself insists that it was a historical event (with binding implications).
Beyond that, it is reasonable that even in the story of the Book of Esther there is at least a historical core, without commitment to the details. The same is true at Sinai.

The tradition itself also seemingly insists that in Shushan there was a historical event that has binding implications, like reading the Megillah, feasting and rejoicing, sending portions, and gifts to the poor. I didn’t understand the point of distinction.

Not at all. It could also be a myth, and people celebrate it for one educational reason or another. But if Mount Sinai is a myth, nothing is left of it.

And what so obligates us to preserve something from Sinai? Maybe that too is a myth for obvious reasons (like the other religions, which are myth). It doesn’t depend on whether we want to treat it as myth or truth, but on whether it really is true or only myth. I truly can’t understand the point.

What logic is there in saying that a story transmitted in such a precise way (a public reading twice a year in which every Jew is obligated to hear every single word, when all communities and groups from every corner of the globe have exactly the same text), while the entire people celebrates in memory of a national event of the nation’s rescue from annihilation, is a myth?

The claim about lack of external sources is unclear. Many historical events are known from only one source, such as much of Persian and Greek history, which is based on the writings of Herodotus, and the history of Christianity and Islam, which is based mainly on the writings of those religions, and so on. (Even in recent history, most Holocaust stories are told by Jews without confirmation from external sources, which allows Holocaust deniers to claim that the Jews invented the stories in order to justify the establishment of the state.) The story of Purim was not a significant event on a global level, but only on a Jewish one, so it makes sense that only they preserved the story.

It’s a little hard to argue that an event in which members of minorities in the kingdom killed more than 75,000 people and took the reins of government under the king within a few days would not be written in the history books, especially under the circumstances described in the Megillah. There is also no mention of a king named Ahasuerus in other sources, and so on.

And as for the Holocaust, just take a short visit to Auschwitz and Birkenau and then tell me afterward whether it comes only from Jewish sources.

Many wars throughout history were not documented at all or have only a single source; there is nothing unique here.
The Jews did not take the reins of government; they were given permission to take revenge on their enemies for several days, and no more than that.
The name Ahasuerus sounds like Xerxes, who is mentioned in several sources.
In the Purim story there is no supernatural event at all, just an astonishing chain of coincidences, so there should not be any difficulty in accepting the story; its source is certainly more reliable than the writings of Greek historians, where it is hard to know what they made up and what they didn’t. It seems there is an anti-religious agenda here that wants to deny all religious traditions as such (they do this to other religions too, by the way).

The arguments against the Holocaust do not deny the existence of Auschwitz and Birkenau; they claim that most of the stories, and especially the numbers, are exaggerated and inflated. In my opinion, the fact that the most enlightened and advanced country in the world established a mass murder industry and exterminated millions of people over several years is far less logical than the story of the Megillah. Reality does not always work according to our logic.

There is also no external source at all for the existence and destruction of the First Temple. Is that too a myth?