Mastodon.art still federates with Scholar.Social, the instance responsible for the death threats.
The Scholar.Social user who made the death threats is friends with the Mastodon.Art admin.
The death threats were specific, and revealed private information about the victim that made clear that they were in serious and immediate danger.
Mastodon.art's admin has a long history of specifically targeting queer instances with lies about racism.
This continues to cause damage to queer communities, which are essential for survival in the face of international trans genocide.
But she does not target all queer instances equally, and uses this in her defence.
If you're out and passing (and usually wealthy and neurotypical), she's willing to use you as a shield instead of attacking you.