Additionally, pissing off CBP may lead to extended delays, luggage searching, and questioning to see if they can find another legally valid reason to punish you for annoying them. And maybe they might revoke trusted traveler program membership due to no longer seeing you as a low-risk traveler. But indeed, they will not finally refuse entry to a citizen.
There are rarer cases where the US government can insist on your cooperation in getting past a PIN, passcode, or password, such as if you show them that an incriminating document exists on your phone and then lock the phone before they can collect the evidence.
And while the exact boundary of the constitutional protections regarding face or fingerprint unlock is not authoritatively settled nationwide in the courts, it’s very likely weaker than for information you hold in your mind like a password.
I strongly suspect CBP can constitutionally require a US citizen entering at an international port of entry to assist with fingerprint or face unlock, though I admit I don’t know how physically they can force the matter if the person refuses. It wouldn’t surprise me if that would be grounds for arrest under at least some circumstances (maybe not all).