The problem is that the right of anonymous speech is a relatively new creation by the Supreme Court (the Supreme Court ruling everyone cites is McIntyre v. Ohio

The problem is that the right of anonymous speech is a relatively new creation by the Supreme Court (the Supreme Court ruling everyone cites is McIntyre v. Ohio Elections Commission in 1995) and there isn't much precedent yet, especially when the case concerns a non-citizen or a port of entry. A more recent ruling in 2008 set some limits on the protection and established some tests for when someone can be legally deanonymized so already there is substantial creep against this protection. Primarily, many rights are relaxed at the border (like search and seizure in Carol v United States) because the sovereign's interests and duties must be balanced with constitutional protections. If this case ever makes it to the Supreme Court i wouldn't be surprised if they rule that the government can require you to disclose all of your social media accounts if you're a non-citizen and any accounts where you do not have a reasonable expectation of anonymity if you are a citizen (I.e., they'd get your Facebook account and any account where you posted identifying details like a personal or work email but not your 4chan account).