Their arguments still hold up pretty well as far as I can tell. If anything "improved" since then, you could argue that what the biggest platforms decided to use (Mastodon, Lemmy) became the de-facto dialect in use, but you still have to explicitly refer to how certain projects do things if you want to implement ActivityPub, which can be pretty demotivating for developers, and doesn't makes the user experience any better.

...and nothing prevents new apps such as Threads from using ActivityPub differently, being incompatible with existing apps and further dividing the space