If internal combustion cars powered by renewable synthetic fuel could be anywhere near half as efficient as BEVs, wouldn't that be a great problem to have?

"Imagine a lossless conversion" is obviously doing a lot of work here, but even if they're only 1% as efficient end-to-end as BEVs, that would still be fine in a world where we had more renewable power than we needed without the capacity to store it all. BEVs would eventually be pushed to dominance by market forces, while in the meantime we would accelerate our transition to net-zero emissions.