Sam, exactly the correct explanation. We just need to remind the readers that we are talking about two-way CO trunks.
The one thing that made GS trunks glare-proof was the fact that as soon as they were seized at the CO end (for an incoming call towards the PBX) the CO circuit sent a ground out on the (normally open) Tip side, as you said. This grounded tip remained until the call was over; then it opened again.
The trunk circuit in the PBX saw that ground, and immediately made the trunk busy to any outbound attempt (an extension user dialing "9" for instance) by the PBX. This occurred either during the ring period, or the silent period, so that the 4-second interval of silence was never an issue: The moment that the trunk was seized, it was busy, and therefore no glare.
Arthur P. Bloom "30 years of faithful service...15 years on hold"
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