I knew Russian state endorsed journalists had been saying that sort of thing, but I thought they had a layer of plausible deniability by not having their actual ministers say things like that. But it turns out he is right, Medvedev did threaten to use nuclear weapons if Ukraine successfully regains the temporarily occupied Ukrainian territory: https://www.reuters.com/world/europe/russias-medvedev-wed-have-use-nuclear-weapon-if-ukrainian-offensive-was-success-2023-07-30/.