Is he bluffing ?

This morning Putin made the implicit threat that he was prepared to use nuclear weapons in his losing war in Ukraine.

In January the People’s Republic of China, the French Republic, the Russian Federation, the United Kingdom of Great Britain and Northern Ireland, and the United States of America issued a joint statement ;

"We affirm that a nuclear war cannot be won and must never be fought. As nuclear use would have far-reaching consequences, we also affirm that nuclear weapons—for as long as they continue to exist—should serve defensive purposes, deter aggression, and prevent war. "

So which is it ; An empty threat from a desperate and deluded leader , or a serious warning that he'd be prepared to risk mutually assured destruction rather than lose the war and most likely his Presidency ?