Why would Buffalo do that?

Over the cap, Foligno doesn’t waive, you cripple your team until players are back from quarantine

Don't know why you think this helps the Leafs.
You have traded to the Leafs, three left wingers, one who isn't good enough to make the taxi squad.
Foligno is nice but doesn't play centre....Leafs will keep Kerfoot and Engvall.
Hall this year likely can't help any team. Heck Engvall is playing better than him
Savard might be better than Holl, but not that much. Then you have the quarantine waiting for this players to get in NHL and Leaf shape, while three regulars have been traded off the team.
Sorry adding these four NHLers to the Leafs and trading three Leafs don't make the Leafs better this year. And the Leafs give up term and futures? Just don't get this logic of the trades.

Hall being better than Engvall?! Analytics doesn’t support it, stats obviously don’t support it either.

Engvall has been playing part time until recently and now seems to secured a centre position Keep Hall and his one goal and his minus play . It's 2021, and the fact he's getting 8m doesn't make him good.
The Leafs don't need wingers and the expense of centres