Google Translate:
First of all, the ActivityPub system is not suited to something like Misskey, which flows at lightning speed.
It was originally designed to connect blogs, small-scale SNSs, and wikis.
How can it handle the TL hell where tens of thousands of requests fly in per second?
It’s based on the idea that it would be nice if various small services could send each other updates, so it’s quite costly.
It wasn’t. Even the very first appearance of ActivityPub in W3C mentioned it being a social networking protocol:
OStatus and Pump.io are both social networking protocols as well.
I wouldn’t say that “social networking” is just syncing blogs and wikis.
The vocabulary is also not that small and the flexibility is also there. It is clearly not meant for just blogs and wikis.
This doesn’t really apply to why misskey was originally created though, which is what this post implies.
What misskey was used for and why it was created can be two things.