Here's the reasoning, should the tweets get deleted: https://thephd.dev/i-am-no-longer-speaking-at-rustconf-2023
How does [person A choosing not to speak at a conference] connect to [person B leaving the Rust project]?
Neither happens in a vacuum. There was likely a lot of internet and real-life drama between people. The basis for the blog post already shows some sign of preexisting drama.
I don't know the context so I can only theorize why they quit.
Perhaps they noticed the drama beforehand and this was the final straw. Perhaps they felt called out by the blog post. Perhaps they saw more internet drama coming and decided they've had enough.
Maybe of note: JT has been very influential in a new language originating from the SerenityOS project, Jakt [1], though his contributions died off over time. Perhaps they're searching for a new challenge and saw this as a good moment to get out?