They did that in 2017-2018. Lost in the first round after Boston blew a 3-1 series lead, only to give up 4 goals in the 3rd in embarrassing fashion.
Then the following season in 2018-2019, they had the 2nd most wins in Leafs history with 46. Blew a 3-2 series lead to the Bruins. Not much progress.
2019-2020, a bit of a worse season. Another "first round" exit to the Blue Jackets.... Definitely no progress...more like a regression.
2020-2021 Despite having their best winning percentage in history (35 wins in 56 games) and having the "easiest" path to the Conference Finals they've had in years, they manage to accomplish an unprecedented collapse against the Habs of all teams. An embarrassing regression to say the least.
So yes, this season they may in fact eclipse their greatest regular season ever in points, and win %, but all that means absolutely nothing if the trend continues and they lose again in the first round. Despite facing a much stiffer opponent than they've faced in recent years. The same argument can be used for losing to a far inferior opponent last season, after having a commanding 3-1 lead.