Well, I have already suggested that without any record or tell-tale of IAS, (or the actual OAT or air density or wind speed around the aircraft) at the time, there is no case to answer - unless a wing or a flap fell off or something? Why not simply show your accuser the derivation of IAS, CAS, EAS, TAS, etc. and ground speed in a text book?
It is not impossible, but unlikely that two aircraft 5 or 10nm apart at the same level would experience markedly different wind vectors, (unless they were crossing a jet stream). You can get different winds at different levels of course, so at the same IAS an aircraft on approach could catch-up with a preceding aircraft on the glide-slope for example.
You would have to check with ATC but they have a mental picture of the winds on their traffic screen, and their screen software can show a prediction of where the aircraft will be in the next 10s, or whatever. We pilots sometimes get asked for a spot wind so they can update their mental model - or we report any particularly strong winds to them.
Beyond that I don't really know what the OP is on about. You sound as if you already know the answer, in which case, why ask us and then shoot our explanations down?
FWIW I was sometimes asked my IAS by ATC when flying C152s, and I remember waiting at the holding point at Dublin, EIDW, in a commercial jet when a poor sod in a C172 who had got lost was guided in to land on the main runway ! He was very apologetic, and Dublin ATC were brilliant - really understanding and helpful.
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Last edited by Uplinker; 25th Apr 2022 at 12:55.
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