Hmm... 120 years ago was 1890... 140 years ago was 1870... whassuuuuuup?
By the reasoning of the Brit archaeologist in the article, that when forts are abandoned in ‘military zones’, it automatically means the builders retreated; so, when the U.S. Army closed down the “Indian Territory” forts, it must mean the U.S. retreated back to the East Coast.
He was ignoring the other possibility that forts are abandoned because pacification made them an expense no longer needed.
There is yet another possibility, though that wouldn't apply in the Welsh-Roman case: advances in military science/technology made the forts obsolete, though a military presence that utilized the new paradigm remained.