No, I'm not mistaken and you've used a terrible example as not everyone has the skill to play a guitar while the majority of players can slap a few keys on a keyboard.
It's not a terrible example at all. I have taken the same approach to the guitar + singing thing personally just as i did with SC2 and it seems to be working pretty well.
Macroing isn't just "slapping keys on a keyboard". You have to not get supply blocked, keep constant worker production, build the right production/tech structures at the correct times. know when to expand, use mules/chrono/larvae etc etc. For you and me this is easy but it's the kind of stuff that people in the lower leagues haven't got a great grasp of yet. I know this, because if they had they wouldn't be in the lower leagues. As I've said before you can get to platinum just a-moving on the minimap if you macro well. If you don't believe me then derank one of your accounts and try it.
If playing SC2 is "slapping keys on a keyboard" then playing guitar is "plucking strings and holding frets". Sure it's easy and any idiot can do it... but getting a meaningful result from it, that's where the skill is.
You seriously underestimate players in the lower leagues and it's changed a lot since you and me were there.
If he doesn't learn from the get go to control his army (assuming he can hit all the correct buttons as he can touch-type/has learnt them) and also macro every attack he does will be an all-in. He won't be able to sustain production at home while attacking and therefore will have no reinforcements.
EDIT: Just wanted to add obviously everyone will improve using different methods but I'm not a supporter of the "Separate each mechanic then meld them later." I think it's stupid and while doing them all at once can slow progress it makes for a better understanding.
I don't doubt the lower leagues are a lot harder now than they were a year or two ago, but while they can macro better this doesn't mean they can macro well. They are still bad. Hell, I'm terrible and screw up my macro all the time and I'm in high dia so I know people lower than me will generally be worse.
In response to your edit - fair enough, it's a matter of opinion and we happen to disagree as usual
but in my experience anything which is impossible to pick up easily, is best broken down into smaller chunks then built up. Works best for me but maybe you are different.