I'm sure that's true, but it's not because the rhizome inherently "needs exposure to air and sun". It's a result of the soil type you garden in, soil moisture, atmospheric humidity, temperature, and likely a number of factors we are unaware of, all combining to promote the growth of the pathogens that cause rot. Getting the rhizomes up out of ground, in your situation, changes enough of those factors to reduce the virulence of the rot causing pathogens.
Rather than say that, people say 'iris rhizomes need to bathe in the sun'. It's easy to understand why they say that
, but it isn't strictly accurate.
