But it's not surprising. Mastodon mainly sees the fediverse as a mastoverse with "other things attached" and gargron mainly sees mastodon as his personal project with "other people attached". It's a bit harsh, but there's enough truth to it, especially for an ecosystem/platform that has as its core ideology cooperative diversity and connectivity.
All of that being said, I think a good amount of blame falls to the protocol. With a good enough protocol and set of guarantees around what compliance with it entails, it shouldn't matter whether someone like gargron behaves in this way (where to be fair, he's built by far the biggest platform over many years now), as compliance with the "standard" protocol should preclude his ability to arbitrarily push things around like this.
But the protocol is very soft. And I think a time is coming, if it isn't here already, where the creators of the protocol are going to rue how much they've left up to developers like gargron who will suck the "standard" out of AcitivityPub. I say this may already have happened already because there are calls (which are very reasonable IMO) to take the mastodon API and basically make it a standard, not least because other platforms are adopting as a de facto standard already. And of course the biggest, sometimes desperate/emotional push back against this I've seen has been from one of the authors of the ActivityPub (evan, if you know them), because there's apparently something in ActivityPub that should be used instead but no body as really heard of and which I've heard is kinda crappy and vague (not unlike the rest of the standard perhaps?).
Bottom line is that ActivityPub needed gargron and mastodon to prove that something good could be made with it and they may regret the bed they've made. What ActivityPub should have done from the start is dedicate efforts to the creation of standardised software, at least for testing purposes, so that there was some sort of standard at the functioning software level for developers to work against. THere are efforts along those lines now, but it may turn out to be too late and one of those things every fediverse developer is "supposed" to use and contribute to but don't bother because they're too busy getting their platform/app to work with mastodon.
EDIT:
And on that note, an interesting project in the fediverse is vocata (https://codeberg.org/Vocata/vocata), because it tries to make the server as generic as possible and instead require the client (ie browser or mobile app) to make all the decisions.