This is a false assumption. Studies done on rats prove this wrong. Showing hgh to be superior for returning normal muscle growth, then the equalivlant igf-1 dose.(These were done mice who had hgh knocked out. Then two groups one hgh, one igf. The igf group return almsot to normal but not quit. The hgh had the same muscle mass as control). They also did this in superphysiological levels and hgh again had a SLIGHT upper hand in growth.(again done in mice with knocked out hgh).
It may not be due to hgh being anabolic(it might by but hasn't been proven either way and really can't be). Rather, increasing igf1 through hgh doesn't mess with mgf pathways. Allowing that to have it's full normal effect. Which I believe to be the biggest reason; as mgf is just as, if not more anabolic the igf. Also by increasing igf1 through hgh, rather than directly, doesn't increase igf1 antibodies nearly as much.
I'll see if I can dig those studies back up. Super busy with school though so it might be a bit. If someone knows the ones I talking about I'm would appreciate if you post them for me.