The submission title does not specify who can take anybody's tokens. Anyone who reads the article can see that the author claims that Open Sea administrators can seize anybody's tokens, and questions whether it is legal for them to retain this ability. This is much more of a nuanced situation than "omg open sea has a backdoor all your nfts are belong to us". The author also says that they will write more about this in a follow-up post.
I believe the author is correct. This is behaviour pertinent to Open Sea's ERC1155 contract (called the Open Sea Shared Storefront), and not their marketplace as a whole.