They used tabuu to map out 100% of the bytes in character pac files, it was not one of them as far as I know.
Furthermore, landing detection is significantly more complex than you all give credit for.
In particular:
Different move types have different landing detection "boxes." For example, PW had looked into it for us a bit, and had found one type location for regular falling with mario, but said he couldn't find all the ones for aerials, specials etc. So its not only trudging around in assembly, but its *multiple* boxes per character dependent on the action they are in.