This is not true, unless I'm misunderstanding what you mean. 1. What is your definition of 'QT', medication? length? observation? Coral/Fish/inverts (some or all)?
2. A QT tank is not 'sterile'. It (should be) pathogen free. Putting a fish in TTM, or an observation tank, does nothing to its 'immune system', unless you are keeping it there for months perhaps. Even with copper, the tank is not sterile, and the fish's 'biome' is not completely wrecked, and the immune system is 'not destroyed'. There are multiple layers to a fish's immune system. One of the most important is just the coating of slime on the skin which contains multiple chemicals that kill parasites and bacteria.
To me it all comes down to the logic of the 'discussion'. Why did the world attempt to eliminate small pox? By some of the logic here, it seems like that would be a negative. How about malaria, tapeworms, syphilis, gonorrhea, HIV, cholera, etc etc - I don't know of many scientists that suggest that its a good idea that everyone be exposed to these things to 'boost their immune systems'. So to me - there is logic to QT of some sort (I do not personally like prophylactic medication). There are also people that do nothing that are successful. I don't see it as a black and white issue, but rather, as
@brandon429 has suggested, I would like to have a better explanation of 'why' one or the other method works. A lot of the 'logic' I read about natural tanks does not make sense (to me).