Well, yes, you're right, there were never millions of ET carts dumped, it was just a part of Atari stock, incl. hardware and software. Apparently, it's common practise for companies to 'just bury things'.
But, of course, this was nice folklore.
As for ETs stock under Tramiel, he was just selling off old Atari Inc. software (and hardware) at the cheapest price. It was just new labeling.