Nukes doesnt mean the end of the world. Remember we've used two already and Japan was able to return to normalcy.
Besides to threaten nuclear intervention after 4 days tells us they're getting their asses handed to them.
Putin is looking for a way out. Plus they announced negotiations at the border. This is a man trying to find a way out while looking tough.
We were the only ones with nukes at the time so there was no MAD. But yes, Putin is desperate since things aren't going according to plan.