Ever since the US made crystal clear that a nuclear strike against Ukraine will mean the US annihilating all Russian forces in Ukraine by conventional means, such small scale nuclear exchange is in fact ruled out. This will in effect invite Russia to simply swallow such a devastating blow, or else end it all. Russia is in fact ruled by rational actors, so it rationally backed off. Thus in effect proving its rationality and the emptiness of its "but what if we're insane" bluff. They're not insane.