He means that even though the probability of going bust is very low compared to the rest of the outcomes combined, which are all successes, you make so little profit from each bet that the chances of hitting the bust probability grow significantly larger. It's not a negligible probability anymore, there's a plausible chance that you'll lose your entire bankroll.
Increasing your EV by multiplying your lost wagers by 100%+x% (because you will not make any profit with a 100% multiplier no matter how many losses are sustained in a row, except for the tiny wager you first used) increases that probability exponentially, the higher you make x.
DarkStar did a really good writeup about it in this thread which I'll quote the relevant parts below:
Martingale 'forces' players to wager more, so casinos get a lot more EV out of their wagering.
To win 1 BTC betting with a 1.2x multiplier you need to risk at least 5 BTC, and you can expect to lose at least 5 times as much as a simple all-in with 2x multiplier (since that way you only risk 1 BTC in total).