I would love to see research that shows that to be a correct statement and why so many in Asian countries do not seem to have compromised immune systems would be interesting as well.
In Japan, before Covid, people did sometimes use masks but that wasn't the majority. Very far from it. If I'm not mistaken it is people who know they are sick that would wear a mask to protect others. That still doesn't work for people unknowingly carrying a virus.
And most people are unmasked: so they're still exposed to at least some pathogens. So I don't see why they'd have a compromised immune system.
I could have misinterpreted the up-thread comment, but I read it as they were saying that the immune system can degrade its available response to certain pathogens when the immune system goes without adequate exposure to those pathogens.
The wearing of the mask is just one piece in this puzzle. Hygiene and sanitary practices vary widely across Asia. It could be that wearing a mask degrades maintenance of the immune system, but other sanitary practices in Asia result in the opportunity for the immune system to maintain vigilance. That is, it could be the immune system maintains itself in spite of the masks, rather than the mask having no effect or positive effect on immune system.
And most people are unmasked: so they're still exposed to at least some pathogens. So I don't see why they'd have a compromised immune system.