It’s something of a moot point, though.

The basic, and well documented current steps are

Depart for Cambodia with a Visa, and funds for the quarantine. (If you’ve got that far, then surely you’ve done some in depth homework, and asked a bunch of questions, whether of the authorities or a forum with people with prior knowledge?)
Arrive in Cambodia, and follow the process. You have to step around the folks who try to argue, circumvent, or deny knowledge.
Deposit quarantine deposit, $2000? (Maybe feels like self funding your own hostage crisis)
Deposit passport. Feels like travel to certain places from about 40years ago
Submit to brain probe.
Sit dejectedly on a bus, maybe buy a souvenir cell sim.
Submit to quarantine for two weeks.
Enjoy freedom.

I’ve not yet heard of anyone wanting to about face and get back on the next departing flight once face with retention of passport. Is it even possible?

It’s shite. It’s their process. It’s their train set- If you want to play; this week, these are the rules. Rail against them at will if needed. Or enjoy 2 weeks quiet contemplation.

It doesn’t much matter what the US, UK or any other government think about it. They’ve already covered themselves by advising against non-essential travel. Which is a fairly broad statement.