Most likely the car you saw was converted from OBD-0 to OBD-1. I do not believe that Hondata makes anything that will work directly with a OBD-0 car. So typically those cars convert to OBD-1 (which isn't that tough) so they can use the Hondata. However, for an IT car, it wouldn't be legal to do that. To answer your second question, Hondata's S100 system fits inside the stock box and is perfectly legal for IT (for OBD-1 Honda's).

This is all "I think" since I don't actually use Hondata, so maybe someone else can verify if this is correct.