Dev docs imply that music devices support live glances and non-music device do not, but this is not true in some cases

Sure I agree. Just saying that the rule of thumb written in the docs was probably true when they were written (except for first-gen MARQ), but not anymore.

Also, just want to confirm that it's true that first-gen MARQ devices don't support live glances, even though they all support music?

I suspect we want to avoid enumerating the devices that support live glance updates here because it will change depending on what product teams choose to support and as new devices are added, so using this music/non-music guideline is a compromise to avoid leaving people wondering how to make the distinction between devices with "ample resources" and those that have "less memory". I ask about it anyway, just to see whether we want to improve this documentation.