Even if they were a fraud, at this point they've made enough money that they could be well capitalized. I'd love to know if they were a fraud (and I suspect they were), but I suspect they got past the "fake it until you make it" hurdle.

Tether had a big chunk of funds seized by a government/bank making them less then whole. They did the needful to get past that and now a distant memory. Scam allegations are made by old curmudgeons who drank the bitfinexed cool-aid.