The list was not public, what are you going on about?
Tesseract is open source, no?
He was also not 'running' anything for most users, any more than Linus is 'running' my pc just because I'm using Linux.
He is running an opinionated client to the underlying ActivityPub network. He states that in user agreement and docs for admins that want to host it on their own.
Do I agree with a design philosophy intended to decrease polarisation? Heck yeah. The amount of downvotes I’ve got shows what kind of people this thing attracts. Reeks of reddit.