I'm just repeating what I've read on here.
I think more likely it's the indication that the chickens had decent living conditions compared to battery hens. And a folk wisdom that they're healthier than sterile eggs.
Someone once mentioned that they're for balut. It's an Asian thing. They let the egg develop a bit then eat it.
There's also the factor that if hens have access to a rooster, their living conditions are probably better than other commercial laying hens.