Wait, so as a naturalized American I could be denied entry to my own country for refusing to turn over my social media passwords?
AIUI, no, CBP cannot deny American citizens entry. They can confiscate your phone (and probably never have to return it) or arrest you, but either way they also need to let you in.
(Though if you're brown, and in certain areas, that arrest can pretty quickly turn into a summary deportation hearing where you have to prove your citizenship without access to lawyer or paperwork. In theory everyone involved would be in deep shit for an incident like that, but in practice they get presidential pardons.)
Also, this article isn't about CBP policy.
They (the government) probably already monitor and record stuff like that, but now are exposing it directly by asking related questions, say "You bought a pressure cooker, and a ball bearings last month, saw it on your Amazon account. What are you planning on doing with those".