This is all elaborated in the article.
Now if, for some reason, too many people decide to withdrawal and cash out on their Tether, there's not a sufficient cash backing there. It'll completely collapse and people will be left with a bunch of Tether that's no more valuable than any other bunch of random sequences of 0s and 1s on the internet. This is no different from the original Ponzi scheme. Just replace crypto with stamps/international reply coupons and it's absolutely identical.