Some people actually think this or Skull&Bones could be a threat to SoT? :D
Atlas, assuming it is indeed a leak of some early concept stuff, looks interesting in scale and could be quite fun if it plays similar to Ark or Conan Exiles. However, the artstyle and technical quality can't hold a candle to Sea of Thieves, it's gonna be janky/clunky as hell and it's gonna be a very different genre. Ship building looks interesting, but it's probably going to require a lot of grinding, and persistent servers means that some dude is gonna destroy everything while you're at work.
Skull & Bones looks extremely pretty and will bring the technical quality expected from Ubisoft. But that technical quality is all Ubisoft have going for them. Gameplay wise, most of their library is bland and unimaginative. S&B looks to be nothing more than a fancy third person MOBA with pirates, casualized and arcade to the max, most likely with a huge microtransaction and lootbox store. Now there might be a market for this kind of game, like there is for most Ubisoft games, but it's entirely different from Sea of Thieves target audience and genre wise, S&B is even more different to SoT than Atlas seems to be, since it looks like you're controlling an entire ship rather than one sailor and, well, it's a MOBA.
Atlas and S&B might sipphon a few players from SoT. It's certainly a good thing that other devs are starting to pay attention to sailing and pirate themed games. But I don't think these two games will pose any meaningful competition to Sea of Thieves. Maybe in 2-3 years we can see a SoT killer on the horizon, but Rare has a massive headstart and Microsoft isn't exactly a small publisher.
I think we'll be sailing the sea of thieves for quite a while. :)