Nu metal stands for New Urban Metal.
Slipknot and System of a Down are not and were never nu metal, they are alternative metal (especially the latter), and Disturbed hasn't been nu metal since The Sickness. Believe and Ten Thousand Fists are just hard rock.
Hell, all these bands (Disturbed as of recently) do guitar solos, and better ones than some 'classic metal bands' might I add. Most notably, Slayer. Kerry King never learned guitar scales, and his solos end up just being random notes which sound like utter dingo.
I feel sorry for half the bands with downtuned guitars that started playing music during the nu metal time, because they were all lumped into the same group. Incubus (whose genre varies - 'Morning View' was pretty much just pop rock) and Godsmack (post-grunge, most notably ripping off Alice In Chains) are prime examples.