Anything that tells you to use the rear brake on downhills you need to realize that they aren't telling you to lock up the back wheel. As you go downhill, weight is already shifted off the rear somewhat. Braking shifts that weight off the back wheel even more. So it becomes easier to lock it up. For a while, many mfrs have been putting smaller disc's on the rear and I think that is why. So they won't tend to lock up so easy.
Though coming from rim brakes to disc, I had been locking up my rear too and went sideways way too many times. Thankfully only one time resulted in a crash. Though all, including the crash were sort of fun and in a childish way I was saying to myself "do it again"! But I didn't.
I think I've got it figured out now, with practice so will you. Don't lock up your rear wheel! But do use rear braking to modulate your speed downhill or anywhere. It's less desirable to have your front wheel lock up and fly out from under you than the rear. IMO.