To be clear, I'm not suggesting the 'in the air' / 'on the ground' signal is a single point of failure for both TCMAs. There would, of course, also have to be a sufficiently large 'mismatch' between measured thrust lever positions compared with measured thrust. An erroneous 'on the ground' signal just enables the TCMAs in circumstances in which they are not intended to be enabled.
My understanding - been wrong many times before and will happily stand corrected - is that idle is only one of the measured thrust lever positions at which TCMA is designed to activate shutdown if the measured thrust is ‘too high’. Apparently there’s an ‘envelope’ of measured thrust lever positions versus measured thrust, outside of which the TCMA will activate (in the ‘on the ground’ state). Consequently, it is possible for TCMA to activate shutdown (if it believes it’s on the ground) when the measured thrust position is ‘more’ than idle, provided the delta with measured power is big enough for long enough. How far advanced the measured thrust lever position may be with potential for the delta with measured power to still activate TCMA shut down? Dunno.
I mention this because I - and know others - are still trying to sort out the implications the various main landing gear truck positions and main door positions after take off in a 78. Maybe there's a weird failure mode in the undercarriage system that keeps the aircraft logic - or at least for the purposes of some systems - 'on the ground'.