Even without a passport? If I rocked up to the border of my country without a passport I'm pretty sure I'd be thrown into a detention centre if I wasn't deported on the spot.

Deported to where? :)

Yes, you cannot be refused entry as a US citizen even without a passport. You will be greatly inconvenienced, but not refused. You may be held in a detention center or holding facility until such time as your identity can be verified.

You can't fly to the US without a passport. They wouldn't let you board the plane without it.

You and OP are both right. No one will let you (as a US citizen) board an aircraft back to the US without a passport (so you'll have to head over to a local embassy to get a replacement if you don't have one on your person), but CBP must (not shall, must) allow you re-admittance at a land border without a passport (at which point they will verify your citizenship through a tedious process).

> You can't refuse to readmit a US citizen (as in a person that can prove that they're readily a US citizen)

Note the: as in a person that can prove that they're readily a US citizen. If you don't have a passport, you probably can't readily prove you are a citizen.

You'd be detained but if you asserted you are a US citizen they are obligated to try and establish the truth of your claim. They'll call family and friends and access government records. In an edge case, I imagine they'd hold a hearing with an immigration judge.

In college, friends of mine frequently went out partying in Tijuana and occasionally lost their passports. They were usually held for a few hours at the border crossing and given a hard time and a stern lecture about responsible behaviour before being readmitted to the US.