Welllll, ya never know. There have been several cases of molly mules (female mules are mollies, males are johns) that have had foals, some more than one. Since breeding isn't what a mule is for, most mollies aren't in a situation where they could possibly get bred, so nobody knows what percentage of female mules may actually be fertile. As the owner of a molly mini mule, I can tell you, she don't know she can't make babies! Nor do a lot of the people who see her. I can't tell you how many times I've had people ask me, "Is she pregnant?" (Yeah, she has a bit of a weight problem)
A mule molly can very well reproduce, it just doesn't happen that often.
It's likely the guy is wrong, but not impossible that he's right.
And if a mule molly with foal really would be worth a million dollars, there's a peasant farmer in Afghanistan who would love to get a piece of that action.