these comics are well known for their alt text

It’s title text, or in web comic circles, hover text. The linked comic’s alt is simply Lisp Cycles.

Pleroma and Akkoma FE can also display conversations as trees.

if I sent you a private message, is that viewable in plain text by not just the instance owner we’re both on

Yes.

other federated instances too?

Not by design, but instances can misbehave.

private messaging over AP

Private is misleading here: for messaging to be private, no third party should be able to read the messages. In practice, this usually requires end-to-end encryption.

Remember when Reddit was the free software alternative?

beat so fire me be rolling in my grave

It’s fake: I see no spider.