I think that was their point

Their point was more akin to “all cities have been abandoned as civilisation ceased to exist”.

If that was the case we would observe a definite decline in population numbers across the world.

We don’t see that which either implies that the existence of civilisation wasn’t the prerequisite for our survival or that civilisation went elsewhere.

I am betting on the second one. By the time Rome become irrelevant we observe the raise of Paris and London.

Or the population dispersed into rural areas and returned to lives of subsistence farming.

Sure, but where? There wasn’t an equivalent of old Rome anywhere west of Constantinople.