"Not being a protocol you tack onto your code in an afternoon" suggests that there is a big hole in the ActivityPub ecosystem; it should absolutely be possible to do that. You can, for example, write a basic HTTPS application server in an afternoon in Python, because libraries to implement all the hard bits have already been packaged by someone else. And similar functionality exists in other languages.

A similar level of cross-platform library support for ActivityPub would greatly ease its adoption across the universe of Internet applications.