Agreed. It's an empty threat. How many dozens of times has that supposedly been the Russian red line, only to find out they are full of shit? Spoiler alert, they are full of shit. They are always full of shit. If Russia is constantly squawking about something, they are lying.
Putin is a coward. He doesn't want to die. Look at his Eternity Tables for any meetings. Even if he did, his subordinates know the war is lost. Anyone who follows an order from Putin to use nukes will be a pariah and tried at the Hague if they ever leave Russian territory. That's if they even survive the retribution.
The West would respond proportionally to the nuke militarily, but we would crush what remains of the Russian economy. Sanctions would make sure no one, but Iran, North Korea and Belarus will trade anything with him. In contrast, Ukraine would get a bunch of fancy new toys.
Nukes work best as a threat, because you can only shoot the hostages once. Use a nuke and Putin is out the last of his leverage. Unfortunately for Putin, he's used the threat so many times, it's lost any value.