This specific behaviour is why I love Apollo. I do this exact thing when skimming through threads -- there's a couple of ways to do what you want to do:

Tap on the top of a comment to collapse that comment and any below it, assuming you haven't turned off "tap to collapse" in settings > general > comments.

Set a gesture for one of the swipe directions to collapse the current thread (I believe this exists, by default, as right-short-swipe) in settings > gestures.