They can't compel you, no. However, they can ask you to do so, and if you enter a privacy password that shows them something other than your real device (which is what this thread was about), you can very likely be faced with charges of obstruction of justice or even perjury. The point was that it's better to simply refuse, and accept the consequences of that, rather than trying to use technical means to trick them or to wipe data.
Note that if you're not a citizen of the USA, refusing to comply with a request like this is very likely to have you sent back to your country and denied access to the USA forever. While they can't legally force you to comply, they can absolutely deny you entry for any reason like this, regardless of having a valid visa and everything else in order. This even applies to legal residents.
