Well you know Petrucci has his own ideas.
My guess is, it's just the mast (basically a reinforced A-Cat mast) that is outside of the F16 box rule.
The X16 has probably a smaller width and is maybe to light.

Bimare would probably produce an F16 compliant X16 if somebody orders one.

Bimare produces there boats during winter.
If you want one, you would need to order it now.