Every good tactic has a way to solve it. That's the beauty of football. Gasperrini's strategy against Leverkusen was very effective. The reason is, he knows that the individual quality of the Leverkusen players is only mediocre, so implementing the highpress man to man marker strategy is very difficult for the Leverkusen players. However, the results would be different if Atalanta's opponents were of the same class as Real Madrid, which has players with more individual qualities.
Gasperini's strategy in the final match succeeded in making Xabi unable to move, this is the first time he has met a team with tactics like Atalanta from Leverkusen's 50 unbeaten matches. Meanwhile, Gasperini is “bored” against teams like Leverkusen in the domestic league. Gasperini really benefited because he already had a "solution" to Xabi's tactics, thanks to Inzaghi's (Inter) tactics being often defeated, a solution was found. because Xabi's tactics are "similar" to Inzaghi's tactics at Inter, even though Serie A is different from the Bundesliga, these tactics work well. This is what I see as why Leverkusen lost to Atalanta even though in terms of performance before the match Leverkusen was the favorite to win.
It is not like Gasperini knew something that nobody else knew, Xabis team did not lose for 52 total games this year, 51 of them in a row. Meaning that if you say that Gasperini knew what to do, you would be practically saying no other team they faced ever knew what to do and only Gasperini did. That is not the case if you ask me, it's football, things like this happens, we could have seen Leverkusen lose to one of the relegated teams and not lose ever again too, does that mean the team relegated knew what others didn't?
Atalanta didn't become league champions, does that mean Gasperini only understands Xabi but doesn't know anything about other managers? I feel like it was just one game, and if we play it 10 more times, maybe Leverkusen gets a 50%+ win rate, but it's just one game and this is what happened.