I think Denji died.
Yoru obviously knew Chainsaw Man ate the Nuclear Weapons Devil, as she talked about it in the first few chapters of Part 2 and set it as one of her goals to get said devil back. When she now says she "remembers it", she doesn't mean she forgot about nuclear weapons sometime during Part 2. Rather, she knew there was this thing called nuclear weapons that Chainsaw Man erased, but she couldn't conceptualise it or remember it in any other way than an abstract word. By that logic, Makima was literal when she said she was the only one who remembered the names of the concepts Pochita erased. Like Yoru, she knew nuclear weapons, nazis and AIDS existed, but she doesn't exactly know what they were. What is a nuclear weapon? Yoru might know it was a very, very powerful thing that scared people. But what did it do? Was it some kind of cancer radiation beam? Maybe a weapon that pulverised matter by separating its atoms? Superweapons powered by nuclear energy? She couldn't tell. But now that Denji is dead, she remembers it all very clearly.
It also more makes sense for Denji's death to have released nuclear weapons from Pochita's than for it to have been reinvented. It'd be awfully convenient for America to have somehow rediscovered them and nuked the USSR right when Yoru needed a power up. But it'd make sense if nuclear weapons returned from Pochita's belly, and like with ears, nukes just reappeared in some military base while the officers went like "oh, shit. We that. Let's use it."
Lastly, Denji dying could be Fujimoto's way to finally wrap up the Power subplot. He dies, goes to hell somehow, meets the Blood Devil (and maybe Conquest?) and comes back for the KoT finale.