I find it unlikely that Tether will start using lightning as its base layer. I've heard them talk about it for years, but the tradeoffs don't seem worthwhile and I haven't seen any updates on the progress. If I was running a business as successful as Tether I wouldn't risk it. Safer for them to stay with a centralized shitcoin they can control because as Shinobi points out here, lightning wouldn't make stablecoins any more censorship resistant.