You’re right. There are multiple definitions of the word stable, and “unchanging” is a valid one of them.
It’s just that every where else I’ve seen it in computing, it refers to a build of something being not-crashy enough to actually ship. “Can’t be knocked over” sort of stability. And everyone I’ve ever talked to outside of Lemmy has assumed that was what “stable” meant to Debian. but it doesn’t. It just means “versions won’t change so you won’t have version compatibility issues, but you’ll also be left with several month to year old software that wasn’t even up to date when this version released, but at least you don’t have to think about the compatibility issues!”
That didn’t take long.