It's a very non-representative, very small sample. The error bars in the statistical inference to the whole population includes both "very common" and "one-in-a-million".
Not every error bar represents a Gaussian, if for no other reason that most error bars aren't symmetric.
The error bars for small sample size relative to population size are Gaussian.
Error due to a non-representative sample can have a variety of shapes, but their distribution might also be unknown. We do frequently, almost implicitly, assume unknown distributions to be Gaussian, but we should recognize that's not necessarily a true fact about the universe.
I think if anything they would be biased towards having fewer allergies than normal people. Which suggests that 0.21% (1 in 500) is a reasonable bound for how rare a moon dust allergy could be.
Never really verified it but I think allergies are more common in developed countries. If that's true, that the data is skewed in the opposite direction