1) You immediately get to see the date range of the thread.
2) You can instantly navigate to any date or position in that range.
3) Thus, it's not really infinite scrolling. At all. The only similarity is lazy loading, for long threads.
4) On mobile, you also get to preview the comments before actually jumping to it.
It's not as awful as I would have suspected from the description, but I still don't really like it. Bar the fancy "scrollbar" timeline they give you to jump directly somewhere in the page - which does alleviate that problem I mentioned a little - it does feel exactly like infinite scrolling to me, I'm not sure where you're making the distinction between lazy loading and this. The content does physically appear later on the page, making the scrollbar jump around, which I heavily dislike. This renders the native scrollbar pretty much useless as a reference point.
The mobile preview is probably a nice touch, though.
Hmm... man I tried to like this stuff and used it with good faith. No idea why I don't like non-static UIs. I just want everything like a static page, almost like a physical analog piece of paper. Nothing moves. The monitor is like a loupe that I scan through on a giant canvas. I can empathize with people that like new fangled UIs, problem is with me.
It's not a competition, though. Nothing stopping a forum from having both features. A forum I'm on has the classic-style forum navigation as well as feeds, and customizable feeds at that. Each user can read through the forum in whatever fashion they like.
Discourse's scrolling timeline is the absolute best UI feature I've ever used in a forum platform. Nothing else comes even close.