alt Hacker NewsTalking and doing are not the same thing. Geopolitics is like Poker, who is bluffing and who is calling it. You are saying only Putin is bluffing - well, I do read russian military blogs/telegram chats. Spoiler: they also think Biden is bluffing.
Don't you see, how this can turn out wrong?
Anything can happen, and people say all sorts of stuff online.
But from the fact that the warning was expresed privately, and using carefully chosen language (unlike Putin's warnings, which are generally aimed at the public sphere, and are full of bluster) -- and considering, again, that the US is fully capable of carrying through with its promise in this regard -- it seems likely the message was received as intended.
Could still go wrong, but the likelihood of things going wrong by not promising any sufficiently serious consequences at all to Russia's regime if it actually deploys nukes seems to be (unequivocally) far greater.