I guess you can remove nutrients if using a skimmer. Why did they say don't use a skimmer ?
It's all depend on each system.
It does remove both trace elements and organic waste in the process.
If you set up a SPS reef tank then some folks will use skimmer to have ultra low nutrition tank.
I'm not going to setup a SPS tank for now and I'll stick with some soft corals, If my system "too clean" they will die out for the most part
I'm trying to get Pulsing Xenia taking over my rock works, they are sucking up Nitrate so if my system too clean there is no food for them to grow
I'll keep my Stichodaclyta Gigantea at the sand/rock interface and hopefully it will become my center piece carpet for this tank, I might add the torch coral or find another Gigantea Carpet then I'm done with my tank and let it grow