They're not airtight in the true sense (besides the helium filled ones nowadays), but every drive made in the past... 30? 40 years is airtight in the sense that no dust can ever get into the drive. There's a breather hole somewhere (with a big warning to not cover it!) to equalize pressure, and a filter that doesn't allow essentially any particles in.

No dust is supposed to get "in" the drive, but dust can very well clog the breather hole and cause pressure issues that could kill the drive.

Unless you’re moving the altitude of the drive substantially after it’s already clogged, how would this happen? There’s no air exchange on hard drives.

Unless your drives are in a perfectly controlled temperature, humidity, and atmospheric pressure environment, those will all impact the internal pressure. Temperature being the primary concern because drives do get rather warm internally while operating.

Sure, it has some impact, but we’re not talking about anything too crazy. And that also assumes full total clogging of all pores… which is unlikely to happen. You won’t have perfect sealing and pressure will just equalize.