The IR signature/target profile of a balloon contraption is very different to that of a jet exhaust, for one. Far less friction heat of movement etc + no engine.
Although the X is an all aspect missile, the parameters would still be quite different.
I think this is would all be firmly in the "not public domain" bucket of details though, as I haven't seen much of this modelled in DCS.
(I suppose they could also have used eg a radar guided AIM-120, but they are primarily for BVR, more expensive at ~$1.1, and far, far more scope for erroneous target locking, so why risk it when the 9X obviously gets the job done. Again, from the point of view of an armchair - or a rig to be precise - sim pilot. I claim no credentials beyond this interest).
