I don't see how you came to that conclusion
@Wayne. Have I missed something? As I read things, C2.29 says a Gun using IFE "may not form/participate-in a multi-unit FG". D6.64 has "The only vehicles ... that may be part of a multi-unit FG are Carriers/armored halftracks, each of which must be CE and
using its vehicular-mounted non-ordnance weapon(s) [EXC: FT, IFE] to qualify for that FG; such a FG may be composed of such Carriers/halftracks and/or Infantry/Cavalry." That says non-ordnance weapons on CE Carriers/armored hts, but not FT and IFE, may be part of a FG. To reinforce this interpretation,
Hatten In Flames has a SSR, "HF11
GERMAN AA HALFTRACKS: Contrary to D6.64,
SPW 251/21 halftracks using IFE (C2.29) may participate in multi-unit Fire Groups (A7.5)."
D6.65 allows a Passenger leader to direct fire of other Passengers including any FG they are part of. It doesn't allow a leader to direct fire of only non-Passengers.
D3.44 says an Armor Leader could modify the IFT DR of its vehicle's MA using IFE (but not if non-MA weapons are added to the FG). D3.4 says an AL or IL could direct certain FGs, but it doesn't mention what may be used so it doesn't affect any restrictions/inclusions in other rules. D6.64 refines that by stating which weapons may be part of such a FG.