It's a really difficult question. It's the million-dollar question, right? I think this is more of a bluff for him right now and a signal that he wants Ukraine really badly and is willing to threaten nuclear escalation if the west doesn't yield on Ukraine.
I doubt that this is over the sanctions, per se. I think he is signalling that Ukraine is really important to him. Ultimately, I think it's a bit of a bluff and there are others around him who may not be as willing to go that far.