No account of this council survives from anyone who claims to have been at it. The earliest witnesses are Chinese pilgrims and translators writing centuries afterwards, working from what they were told in monasteries in the north-west, and they do not agree with one another about the basic details. Where it met is the clearest example: the sources that name a place do not name the same one, and no site has ever been identified on the ground.
That leaves historians with a choice between two readings and no decisive evidence for either. On one, a king convened an assembly which produced the Mahavibhasa, and the details blurred in transmission the way details do. On the other, a school's collective work over generations was later given a founding moment and a royal patron, which is a thing traditions do routinely and which is easiest to do once everyone who could contradict it is dead.
The Theravada tradition does not recognise the council at all, which is not evidence either way: it had no reason to record a northern school's assembly and every reason not to. Nor is the absence of a site decisive on its own, since a great deal of the archaeology of Buddhist Kashmir and Gandhara has been lost to later building, war and looting.
The atlas records this as unsettled rather than choosing. It also records the location as a genuine gap rather than a disagreement, because no scholar defends a site: the question has not been answered and it has not really been fought over either.