You drastically over estimate the number of people who were creating content on Reddit leaving for Lemmy.
The entire population of Lemmy is smaller than a mid sized cat sub on Reddit and the only content that is being reliably generated is memes.
Yes, a few subs did have their communities split, and some left permanently. But if you think Reddit corporate got at all bothered by Lemmy’s MAU numbers… no, they’re not that great.
A bunch of mods did indeed stop activity on Reddit, but all in all there’s been no practicable drop in volume.