That's a bit of a myth, or maybe oversimplification would be a better term. There's nothing in the physiology of an iris rhizome that would indicate that it needs exposure to air or sunlight. It has no chlorophyll, for example. The rhizome is essentially just an energy storage organ.
I suspect the idea that the rhizome should be partly exposed above ground was originally promoted by commercial growers as a simple and effective means of preventing some of their customers from burying the rhizomes as if they were tulip bulbs, which would clearly cause problems because it puts a significant portion of the leaves below the soil line and leaves obviously aren't adapted to that. Just a guess, though.
And, of course, irises generally grow just fine with the rhizome partially exposed, so there's little harm.
Many funny ideas get passed around like that. When I was a kid, my Mom and grandma always cut their iris leaves back to six inches or so in early July, and so did everyone else we knew. Why? Because the orders from Schreiner's arrived in July with the leaves cut back.
