According to the modelling I can find, yes, the conservative party would have won the most seats in 2021 if we’d had a more proportional system. I goofed, FairVote actually has the cons winning more seats under STV, but the liberals more under MMR.

But critically, it does not mean that the Conservative Party would have formed goverment, because under a more proportional system, they would not have had the seats to form a majority.

They would have been forced to either build a coalition with another party or parties, or they would have had to allow a majority coalition to form government if they were unable to make enough concessions to do so.