The person that found this is a hero.
Whenever I see slightly weird behaviour, there is a temptation to just move on because there isn't enough time, running software is complicated, and there is something else I want to do. I will try to change my attitude in future in case it uncovers a backdoor like this -- it would be educational too.
Despite using Tokio underneath, I think that Actix does NOT do work stealing and uses mostly separate threads:
Given this architecture, I think the article might inaccurate when it says that Actix handlers must be Send + Sync. See also: https://www.reddit.com/r/rust/comments/14cbe1u/why_does_actixwebs_handler_not_require_send/
Actix is a bit weird, but it has been around, and used in production, for a relatively long time.