The rationale behind the “non-airtight” for me is that
-there are a couple of extremely unpleasant experiences from e4 members that tried storing cards in a fireproof safe o similar boxes: what it could happen is that wet hot air is trapped inside and can quickly condense in the winter, warping all the cards (even the graded ones)
-I saw an interview with a museum employee who explained how to properly store comics and manga: their thesis was that the paper/inks release volatile compounds and acid with time, and being trapped they can discolor or change the cellulose properties. How and if this translates on TCG cardboard isn’t proven, but I would avoid it.