I don't think he has officially said one way or the other yet as it will be something to be clarified in the manifesto?
I know there were some references a while back to a "spokesman" saying Starmer has a long standing opposition to PR, but have not seen anything more concrete. But if you have a link where Starmer himself has definitively said no PR I will stand corrected!https:
Eta: I found this from over a year ago, so I guess you are right. Which is disappointing. But maybe he has changed his mind, you never know...
www.theguardian.com/politics/2022/sep/24/keir-starmer-defies-call-for-changes-to-first-past-the-post-voting-system