I really don't think this holds water. The idea that those making the decisions were like "oh, this might be a bit of a shitty run for the leader, maybe we'd better give it to a woman" seems like a reach.
It's another way Labour voters try to maintain the narrative that all Tories are racist sexists. I don't think it would matter how many women, or non-white leaders they have, Labour people will continue to try and convince themselves that of course the Tories are sexist racists, and hate poor people, and hate the environment, etc.
It's how they maintain their own view that they are Good, Caring, and Kind.