“I’m no supporter of Biden, but that’s actionable. Threatening the sitting president with a coup?”
First of all we don’t know if any of this is true but even if true - the 25th amendment is constitutional. Not a coup.
Biden should have been removed via 25th amendment long ago. Both sides didn’t want to do it. Dems thought it looked bad politically for them and the Republicans didn’t have the power to do anything about it but even if they did have the power they preferred to run against a damaged President.
If that is in fact what they threatened him with, sure.
As for when, he should never have been sworn in.