An incredulous anachronism?

I was struck by this use of incredulous in an old trailer for the 1932 Universal Studios film The Mummy, starring Boris Karloff:

My first reaction was that it should be incredible – since incredulous means ‘unbelieving’ or ‘disbelieving’, not ‘unbelievable’ – but it seemed unlikely that such a ‘mistake’ would have slipped through unnoticed. So I looked it up.