Well Undella's also another place they could've settled, given how you mentioned Abyssal Ruins are right there and it might've been a stable location to settle in - food to eat nearby from fishing, sun, etc.
As for the items, maybe they were natural and not necessarily human owned? I mean they could've been from a past... settlement, even before being officially discovered.
I think that's what he's trying to say. Like, Native Americans and the Europeans that colonized North America. We "discovered" America, but the Native American tribes had already lived there for like 15,000 years.
And considering...200 years of recent settlement...ancient civilization...based on New York...doesn't seem that too far-fetched, no?