I agree the optics of this aren't great, but the real question is if this higher follower count on Mastodon translates to post engagement...
My brother once bought me followers on Instagram as some kind of joke and that did exactly squat for reach as far as i could tell. Not saying Proton bought followers on Mastodon, but I am saying followers don't necessarily mean anything.
Mastodon peaked a couple of times but then usage dwindled because people started using Bluesky or returning to bigtech platforms because their buddies didn't move over. I suspect a fair amount of these mastodon subscribers are stale accounts and the Proton team just aren't seeing the engagement they have on the other platforms.
The best option here is to just tank Chrome's market share instead of making something that's obviously not ideal, work.