How is creating a new Activity type preventing compatibility with the rest of the Fediverse?

If they chose to use any of the 3 solutions I proposed, there would be no changes on the other servers to receive and parse the message. But because it uses a different type, now those serves that want to store the information about an answer being accepted have to write code specifically to handle messages from PieFed.

It also works in the other direction: if I want to send an “accept” activity for a comment, I could do it from my server and PieFed could easily understand it as well. But because they want to create their own ad-hoc solution, then they won’t be able to.

When you’re the first one doing something with ActivityPub, you have to create it yourself.

No, you don’t. The whole point of Linked Data and RDF is that nodes can send data to each other without having to agree on any new protocol

Piefed features inspired by existing implementations such as the emoji reactions

You are only making my point. Emojis have already a defined extension, this is why it’s easier to adopt it.

picking the one feature

It’s not just that. They also proposed some ad-hoc activities for moderation in the past and their “import community” works by taking posts and rewriting them as if they originated in the piefed instance. These are all signs that the devs either don’t understand or don’t care about JSON-LD as an standard.