The spawning rules are totally independent of the transport rules and have no relationship. I can understand why players in 5th edition might explain the rule to non-Tyranid players by comparing it to the old rules for disembarking a transport, although the two were always different, since in 5th ed you disembarked up to 2" away from the hatch*, as opposed to 6" for Tervigon spawning placement.
Nothing has changed about the spawning rules. They still state clearly to place the termagants so that no model is more than 6" from the Tervigon, so you're going to measure that from the closest point on the base of each.
*Using the same rules for measuring distances, which have not changed from 5th to 6th; closest point to closest point.
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