Not in policy, no he definitively isn't. It's actually possible to join and have a career in political party without believing a word of what they stand for. There are examples all over the world, but Starmer is among the most blatant ones.

My best theory is that he's chum with one of UK's many spy lords - that is, upper class twats with a sinecure in the secret services - and that he's trying to destroy Labour for them (and the good of the country of course). He only failed last election because the conservative party was even more self-destructive.