What I think MacGirvin's trying to convey overall is that protocol standards are often:
a) too vague
b) too specific
c) not strictly followed, both by accident and on purpose.
And thus they don't work as well as they might.
The #Pleroma author and others apparently concur: #ActivityPub has some of a), #Mastodon does some of c), and this makes their work harder and less effective.