They discovered what they're calling "the paved area" when they literally drained the swamp and under the stones got plant matter that dated to around 1000 AD. I think range might have gone up to 1100 or 1200.
When you consider that an ancient Viking settlement in Nova Scotia dated from around 990 to 10-something, it's really not at all far fetched that other Europeans thought this might be a good place to hide shit.
I mean, for how long was the prevailing belief that the first Euros to land in N. America was Columbus, et al.?
So for the 500 or so years that the knights templar would have been active - before Columbus - N.A. was virtually terra incognita.
And if you think the Templars were wiped out in the 14th century, guess again. They maintained a presence in Portugal for a few centuries post.