Actually, that got me curious. Couldn't a billionaire crush tether by shorting so much of it that it exceeded the cash reserves?
Short answer --- no. They can just print more tethers --- which exchanges and crypto-fools will readily accept as being worth $1 USD.
You're making a common mistake --- you're assuming that the crypto market is actually a "free market" similar to the regulated stock and currency markets. You're assuming it is free of the inside manipulation that keeps the price of USDT artificially pegged at $1 USD no matter what.