The facts of the matter are: a) the LDs and the Greens need PR to make any headway in parliament and b) the Tories are never going to allow PR.
Therefore, the only chance of LDs and Greens getting it are to help elect a Labour government. It will be worthwhile standing down in a few seats to...
There may be one or two but that's not really the point. What LDs and Greens need is for PR; they're going to be doomed to a handful of seats until the end of time if we don't ditch FPTP. I'm sure they'd be quite happy to drop out of some seats that they're not going to win for a genuine chance...
Which is why I mentioned earlier that Starmer may be more inclined to look at an agreement with the LDs (and the Greens). A promise of guaranteed support for PR in return for LD/Greens standing down in certain seats may well be viable.