Possible

IC.

Photo evidence? Anything more than just him saying so? If he wasn't doing gymnastics what did he have a coach for and why was he involved in a school show? What kind of coach would ask someone who's never done an iron cross to perform one?

I have heard people claiming such things before, and no, I dont believe it. One old guy told me that by doing several pushup sets per day, you should be able to do a butterfly mount on rings in a couple of months, which of course is utter nonsense. I also heard a friend tell me that noone in his class was able to get highest grade in PE class because none of them could execute a cross, which of course is RIDICULOUS. It truely tells of the ignorance which exist in the world of training. Right up there with "toning" and "fat converts to muscle" claims. It is more likely that he did something a different skill, like a normal L-sit in support, and childeren do have problems differentiating similar things from eachother. A proper L-sit on rings is alone too hard for 99% of untrained 9 year-olds.

Called.

The only possible way that your father-in-law could of done such a thing is if he has an extremely rare genetic condition (only a few confirmed cases in history) in which he does not produce myostatin. Even then, it is a stretch and it would probably had to be a sloppy bent arm cross.

We have a guy in our club that did the IC when he was 10 years of age. He lost it when he got into his puberty.

Was he trained as a gymnasts though?

Yes, but he could do the cross very early.