Okay, so I got a bit sidetracked upgrading Gitea and doing a few other things, but after poking around in the code a bit, I've realized...

Abstracts aren't just a weird broken equivalent of content warnings. They're actually for defining different summary texts on different networks! It just so happens that the summary field in ActivityPub is commonly interpreted as a content warning, so that's why an [abstract] in my post gets displayed a content warning on everywhere except Friendica.

Personally, I kind of like [spoiler] better, because it can be used to content-warn a section of a post, but [abstract] is what everyone else uses (and what Friendica automatically inserts when I reply to a CW'ed ActivityPub post), so I'll implement support for always displaying [abstract]s as warnings first.