sure, but coalitions do happen under FPTP, and from 2015 to 2024, when the SNP were the 3rd largest party and the tory vote was yet to collapse, labour could only have formed a coalition, or a minority government propped up by the SNP. Despite this, they ruled any deals with other parties out.

besides, like you said, labour have no intention of bringing in PR, which means the next general election will be a FPTP one, so his willingness to work with starmer bears no relevance to his position on PR