Well yeah, replying in-line to the patch to make comments is definitely a requirement. :-)
You always reply with quotes; and you always reply inline, below the thing you're replying to. Quoted material is typically indicated by '>'.
So for one thing, you can easily trim your reply to just the bits of the patch that you think is important; that makes it easy for anyone reading; you don't have to skip over large parts of the thread.
Then you can also re-arrange what you're replying to, to make it make your reply make more sense.
Furthermore, suppose A replies to the patch and makes comments 1, 2, and 3 (inline in a single email). When person B replies to A's email, they typically also send a single email, perhaps with replies 1' and 3' (trimming out comment 2, which they don't care about). This means if you've read A's mail, you can just read B's mail, and all the "new" comments are collected in one place (with the thing they're replying to).
Contrast the above with gitlab, where A's replies 1, 2, and 3 are spread throughout the patch; and B's replies will also be spread out, meaning you have a kind of discoverability problem to see both 1' and 3'.
And suppose in the course of talking about something you realize this bug may actually be a security issue -- you can remove the list, cc' "security@", and pick up the discussion without having to do any copy & paste. Same goes if you want to make a private comment, or bring something to someone's attention; just reply, remove everyone else, and say "FYI".
I wish I had a really good recent example to show you, but a quick skim of today's threads isn't anything special. :-)
I mean, I was arguing with people online [1] on BBS's in the early 90's, and I was on Usenet back when that was the main social network; so this "reply and comment inline" way of having a discussion is pretty well ingrained for me. But there are people on my team in their early 30's who also took to the email workflow really quickly too.
[1] https://xkcd.com/386/
