immune i feel is an overstatement. less prone to i think would be a better way to state it.
i did not QT my mandy. i just rolled the dice with her. i didn't like doing that, but it was my only real reasonable choice.
they don't tolerate many medications well, so medicating them is usually out of the question.
it would be possible to QT them, but only really if you had a fully cycled and mature QT system, with enough pods for them to stay fed for the duration of the QT period. you could of course supplement with other foods, but their primary food source would need to be the pods.
so, is it possible? yes. given the right setup and circumstances.
with that being said, in most cases it is not really feasible.
i would never outright tell someone not to QT something, but in this case it is a calculated risk for the overall health of the fish.