I agree. As much as Labour screeched for a GE when the Tories changed leader / PM, the only people who actually voted for Starmer are those in his constituency. Many voters vote for a party, not a PM or even an MP.
It's a total nonsense to use a potential GE as a threat. However...Starmer strikes me as the type of man who'd go nuclear if forced out, so it wouldn't surprise me if, when he went to the King to resign, he asked for permission for a GE too. Might as well go down in flames. 🤣