There has to be a balance between weakening Sunak and losing people on the left of the party though. I'm on the fence about voting for labour at the GE but shit like this really makes me distrust him and the party.
This isn't just about 'weakening Sunak', it's about winning a big enough majority to actual make real changes to the country. Labour identified that their poll lead was stacking up votes in areas they were already likely to win. This is about appealing to people in other areas.
There aren't that many seats where Labour will lose because people on the left of the party refuse to vote for them. (and there can't be many on the left who, when it comes down to a close fight between Tory and Labour, won't be voting Labour).
There are, however, plenty of seats they won't have a chance in, if people think they're too far from the centre. This kind of stunt helps target people who think a Labour government will be too left wing.