"It winds up you lefties"

Hardly. From a purely tactical point of view it's a dream result for the Labour Party. What would have been bad news for them would have been for Boris either to win with an overwhelming majority, or for him to be voted out.

Chucking an unpopular leader in the way the Tories do every now and again means that the replacement can get a boost. It worked for John Major, and it worked for Boris Johnson back in 2019, and in both cases they won the next election.