That’s exactly the point?

He isn’t threatening Russia with getting nuked, he’s threatening NATO with HAVING nukes.

If Ukraine beats back Russia to the border, things can end. Ukraine can join NATO, Ukraine gets its security via NATO and Russia will be sore, but accept Ukraine doesn’t pose an existential threat. Peace and normalization can eventually return.

If Ukraine can’t beat them back, and can’t get the external help it needs to do so, yeah they can build some nukes. They might have to use one to show they’re not fucking around. Suddenly Ukraine IS an actual threat to Russia. Suddenly this isn’t about prestige and empire building anymore. It might end the war but normalizing relations and the future of Europe is very unclear. It would make NATO membership impossible. This is worse for NATO than Russia taking Kyiv.

Ukraine wins either way. Russia loses either way. But NATO loses if Ukraine gets the bomb.