It's more that it's more evidence towards it not being any one or few major disasters that caused a rapid abandonment as always suspected but just the slow, eventual decline of people moving elsewhere, not very exciting or newsworthy but sensible.
Sounds like maybe they're also suggesting, "Perhaps we don't need an environmental catastrophe, maybe it's more like what happened to Detroit."
Which seems pretty parsimonious to me? Without so much fixed infrastructure such as harbors and paved highways and the Ford Rouge factory, there's less to keep pulling people back to an urban lifestyle if that's not what they're after.
A couple of really cold Winters or hotter than normal Summers, or competition from other animals, including insects, crop disease, etc., can drive any population out to seek new land to conquer and inhabit. Also just internal disagreement and the braintrust left, the ones who stayed were left to fend for themselves...