Account portability is in no way incompatible with ActivityPub. It's not built into the spec., but it's also not forbidden / prevented by the spec. in any way. From ActivityPub's perspective it's an implementation detail.
Would it be nice if it was built into the spec: yes. Does that justify throwing out the baby with the bathwater?
I was hoping pfraze would offer some better reasoning, but while the post above is a great read for the technically curious, its very "in the weeds" so doesn't really address the important high-level questions. Textbook "technologists just want to technologise" vibes: engineers love reinventing things because working out problems for yourself is the fun part, even if many people have come together to collaborate on solving those problems before.