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.



I saw a separate post linked in the Reddit thread on this, where it was said.
Source for the full quote of link + translation (Reddit alert)
Don’t know more information than this. But the wording there suggests to me that what they’re switching to isn’t fundamentally incompatible with the fediverse. They seem to be treating loss of compatibility as an allowed risk, not a guarantee. So they’re probably still using ActivityPub as a foundation for what they’re building.