That's just not true, though. Starmer has only increased Labour's vote share by 1.7%. He didn't significantly outweigh the loss in votes by demotivating opponent vote. Tory voters were more likely to vote for Reform or the Lib Dems than stay home.

Labour's opposition not being united is probably more attribuable to the Conservative party itself not being united. The amount of voters leaving the Tories to go to the Lib Dems and Reform has steadily been increasing.

Besides, a significant part of motivating people to vote against Corbyn came from Starmer's wing of the party itself.