I think I misunderstood the original comment. I interpreted it to mean that health agencies were deliberately understating the effectiveness of masks to reduce the number of people who wear them. I was more looking for a source on the "deliberately understating" part, but on re-read I don't think that's what the OP was saying.

I don't think I have seen an official source saying "we're deliberately understating the utility or masks, so that care givers can have the limited masks". Instead, there are a lot of communications where their utility was understated; and at least I assume that was a deliberate policy, because that's the most positive thing I can assume about their motives.