Thanks for the visual example. I was also looking for this functionality and was about to ask, but I just figured it out.
For anyone confused, you hold down on the top part of a comment next to the commenter’s name and it should collapse that chain.
Personally I would prefer just tapping on the colored lines on the left side, at least that’s what I’m used to.
Beehaw actually federates with lemmy.ml, but blocks federation with lemmygrad. In fact I believe that is the only instance that is blocked on Beehaw.