I dunno. Notice that the expert simply shows the existence of word usage surrounding "arigato", but he doesn't say something like, "we have in our hands documents with 'arigato' being used as thank you from pre-date-X". If he'd said that, I'd totally agree. But since the argument is framed around the existence only of related words, I'm troubled. To play it safe, if I thought it was a big issue (which I don't... it's just a curiosity to me), I'd leave it as "debatable".
But thanks a bunch for the contributions.
Mike