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.