I don’t think we found any specific groups of people attacking Lemmy. I personally just saw one or two what looked like individuals trying (and succeeding) to take Lemmy down with a few very simple requests that forced Lemmy to do lots of compute (something like fetching the next million posts from page 10000). The fixes for those were simple because it was just missing limits checking.
I’m not sure if there actually was a larger organized attack. Lots of performance issues in Lemmy simply appeared simultaneously and compunded each other with a rapidly growing number of active users and posts.
Interoperability is great, but sadly there isn’t really any organized group effort to standardize more aspects / extensions of ActivityPub. AP is really “thin” in that it barely prescribes anything. There’s not even a test suite to test whether software complies to the spec of AP.
So everyone kind of does their own thing, and fixes interoperability on a case-by-case basis. This makes it kinda frustrating to spend time on - lemmy already has special cases for many different softwares (peertube, mastodon, …) and every one increases the complexity.