He spends them to support his other projects. And in a moment, if everyone wants to withdraw their USDT, Huobi simply won't have the money to process the withdrawal.
I've always been suspicious about Justin Sun. He's just another con artist, just like all the other big crypto "whales" like Sam Bankman-Fried or the guy from Terra/Luna(I forgot his name). The crypto industry needs to get rid of centralized exchanges and their shady CEOs. DEX are the future. Cold wallets are the future. I don't think that USDT will face difficulties because of this case. Huobi might have to shut down, but who cares at this point(except the people, who have money in Huobi). The crypto world will survive without another centralized exchange platform.
I also expect Binance to shut down at some point. The crypto winter will continue in 2024, but the bull run in 2025 is going to be huge.
Decentralized exchanges have some problems that do not fit well into the regulatory policy of market regulators. For example, on a decentralized exchange, it is impossible to block an account on demand, request data on transactions of a particular user, etc. actions that work in case of "special situations" or for "special persons". Therefore, in my opinion, they will be in disgrace, or at least with a bunch of restrictions and rules that will nullify all the advantages of decentralized exchanges