They dated the piece of iron through a mark the maker had stamped into it. Without going through the video again I couldn't tell you what the 'exact' date was.
They confirmed the maker's stamp by examining tools made in the period in a museum in the Basque country that deals in such stuff.
They then examined documents drawn up officially at the time describing what the Basque fishermen were doing, where they were doing it and how they did it. They state quite clearly that they were trading with the natives.
If you watch the video you will get more out of it than my half remembered ramblings.