It would be a serious problem if hydrogen were used for diurnal storage. But for rare event backup, efficiency is really unimportant, since so little of the energy produced will go through that channel. What matters for use cases like that is minimizing capital cost, not maximizing efficiency.

I'll add that the alternative proposal, excess renewables with curtailment, has 0% efficiency on the curtailed power. Using that power for anything at all, including making hydrogen, is more efficient that just dropping it on the ground.

Hydrogen is so useful for so many other things that its (present) poor round-trip efficiency doesn't matter much. And, that efficiency will only ever get better, not worse, and will get better faster the more of it there is.

But hydrogen is very far from the only storage choice, it is just one that is attractive for numerous ancillary reasons: we need to make the hydrogen regardless, so might as well store it too.