Signal grounds can be isolated from safety grounds. For a single-ended connection it depends upon how the driver and receiver are designed; you can isolate the ground of a single-ended signal from earth (safety) ground and break a ground loop. I have done it, many have, and a number of consumer components do it. My old ARC preamp did it, my new Rythmik sub does it, for example. Not being an audio tech for years I couldn't say what they are all doing now. It is usually done with a low-value resistor that makes the signal ground path a lower-impedance to the signal path than the safety ground and thus "breaks" the loop.
The purpose of a twisted pair is to minimize coupling from common-mode noise and, in some cases, provide a more predictable controlled impedance. It is often used in a differential design that offers the ability to break a ground loop but those are separate things.