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 tap and 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.
I see you from the Beehaw lemmy instance!