He mentioned that they will only honor the multiplier that can actually be won on those slots. He also gave an example of some guy that won a huge amount on a slot, but it was only a glitch in the software, so the casino voided the win. (Example : Max multiplier for the slot was 5000x but the win showed 50 000x)
Going back to what stake does with third party games with potential winnings that are higher than the max win on stake, I personally have never tried betting on any third party games, but I think in a situation like that, stake is supposed to have a systems that checks and ensures that bets coming from third party games have a win potential that is withing their normal winning limit, to avoid possible problems or misunderstanding between players and the casinos just incase luck hits and some one wins.