Well the guide only speaks of how it's unsafe to put the device to sleep mode so I'd say yes. Though admittedly I've unmounted my SD card while the console has been still switched on in the past and nothing's happened, so if you're afraid you could do just that. If you do unmount while it's on though, make sure you run the test to check that the SD card is not corrupted.
Tbh I never understood why the guide wants you to keep unmounting and mounting the SD card all the goddamn time. Why can't you just put all of the files into your SD card at the start of the guide and cut down the times you need to unmount significantly -_-