True, it's really impressive work. It might not necessarily be defined as next-gen technology though?
I believe they did the traffic and pedestrians through the Niagara VFX/particle system, which was introduced in UE 4.2X in 2018 (to replace Cascade) and works to some capacity on all platforms UE4/5 supports. This is how flocks of fish and birds are managed, you can also do armies of soldiers in this way by tracking masses of independent objects as "particle" behavior, and then you can of course do effects and things which are less tangible and more what one would expect from a "particle system".
The ability to manage that many cars and people without having to drop them into low-fi models or sprites or even fading them out on the horizon, that's pretty amazing, and that's Nanite (at least, for the cars) doing the impossible rendering of all of these cars all on screen all at full detail all the way into the distance of the highways. Then just the sheer amount of cars and people is I would assume beyond what an old console could handle on a good day? It's a great flex of these new consoles and high-end PC hardware. It's more volume and lack of fidelity compromise than shall we say "next-gen magic" making this possible, though.
As I've said about other topics on this thread, I believe the missing wow-factor of next-gen consoles isn't necessarily because these new consoles have been held back from accomplishing something never possible before because they're anchored down by barnacle hardware. (There's some of that, but that factor is IMO overblown, as much as I hate to say it. I was bullish on cross-gen-never before the gen started, but the more I come to understand scaling approaches, the more I get how even cellphones can play apps previously only possible on ultra-high-end systems not too long ago.) Rather it is more to do with new features/technologies coming late into maturity (Niagara came out of beta and certified production-ready in May of 2020; UE5's Lumen and Nanite were still classified as experimental and early-access until 2021; Chaos Physics is still considered to be in beta; and that's just UE5 features, there's a lot else out there swirling around in experimentation but only solidified in usage in professionally-produced games here and there) and this wretched plague (and unexpected complications of development... and bad management) pushing back everything that was on the leading edge of technology. 2023 should be to be the second wave of "real next-gen" if you will that 2022 or even 2021 should have been, and some of it is even going to come in cross-gen flavors as well as in next-gen exclusives.
Small correction: you apparently would still have different LODs in a Nanite environment, they're just included as steps in on the detail scaling approach. Reflections also isn't solved just by going to UE5, in a Lumen environment would still put upon you the same choices (with the same positive/negative aspects) of SSRO or raytracing or whatnot to make reflections. And then, baking in lighting... phew, that seems to be a toughie with graphic technicians as far as how much Lumen can or can't handle the demands of an entire scene, but basically it's a tool and it;s a great tool but it's not the one ultimate tool which makes all other tools obsolete.
...I'd really like somebody smarter than me to come between us and say what it's like actually working with these technologies (I am not a game designer) but I think at the very least it's fair to say that the promise of UE5 solving all of the problems of game development is a little bit of salesmanship.