Several factors could cause the time to the true MRCA or IA point to depart from the predictions of our model. If a group of humans were completely isolated, then no mixing could occur between that group and others, and the MRCA would have to have lived before the start of the isolation. A more recent MRCA would not arise until the groups were once again well integrated. In the case of Tasmania, which may have been completely isolated from mainland Australia between the flooding of the Bass Strait, 9,000−12,000 years ago, and the European colonization of the island, starting in 1803 (ref. B13), the IA date for all living humans must fall before the start of isolation. However, the MRCA date would be unaffected, because today there are no remaining native Tasmanians without some European or mainland Australian ancestry.
No large group is known to have maintained complete reproductive isolation for extended periods. The populations on either side of the Bering Strait appear to have exchanged mates throughout the period documented in the archaeological record. Religious isolates such as the Samaritans occasionally have absorbed migrants from outside the group. Even populations on isolated Pacific islands have experienced occasional infusions of newcomers. Even if rates of migration between some adjoining populations are very low, the time to the MRCA tends not to change substantially. For example, with a migration rate across the Bering Strait of just one person in each direction every ten generations, rather than the ten per generation in the more conservative simulation described earlier, *T**n* only increases from 3,415 years to 3,668 years.
Based on the above assumptions, the authors conclusions do make sense to me.