That's the nuance missing from the parent's snark, masks are most effective at preventing the wearer from transmitting infections to the people around them (especially important in an operating theater). Masks may also help prevent the wearer from inhaling airborne pathogens, though they're less effective there.
Also missing from the discussion is that it is easy to prove that an N95 mask works because the effect is so dramatic.
The fact that the efficacy of a surgical mask is more difficult to prove does not mean that it doesn't work. And, as you point out, the major benefit is to the people around you so that you don't unintentionally spread the disease before you realize you have it.