Chris J Dixon wrote
jim wrote

"All units are airtight and virtually watertight."

That's OK then.....

Am I alone in thinking that air, being thinner,
would be less likely to leak through than water,

Unlikely you are the only one likely to get that so wrong.

or is it more complicated than that?

Yes it is. Most obviously with PET bottles of stuff like beer.

The CO2 will eventually diffuse thru the PET and the water never does.

And with plumbing, water and **** laden water etc can see
that blocking small unpressurised leaks whereas air doesn't
do that with leaks as much.