I think I agree with you almost fully but not quite. Systems design in the sense of 'designing systems' means that a lot of the time you are dealing with glue, but perhaps quite a bit of time is spent on the 'blueprint'. A good system design means that you can synthesise your learned knowledge with that of the body of knowledge to produce a new solution. So sure, you from experience know that a doesn't go with b, but if you include c, you can z. But sometimes you need to break things down further, to components you have haven't used, have never seen before, and only barely understand. This is where your CS first principles come in. You need a URL shortener. Should you create a hash of the url on demand when requested? Well maybe not, because you could get collisions, how about pre generating and picking them up at request time? Oh look, someone has written an article on this problem!

So much of system doesign is understanding what your 'actual' problem is, and then thinking it through, looking for what others have done, and then bringing it together with your team knowledge.