How is that a benefit over ActivityPub? Hard-coding a post size limit in the protocol seems like a negative thing to me?

Because it's a microblog like Twitter.

I'm still not clear on why that's a benefit?

Nothing about the architecture of the protocol itself? Or better account migration or improved design of name conventions, or improved server performance?

The mastodon server I use has a max length of 5k characters, so not really.

That's not part of the ActivityPub protocol.