You misunderstood him. What he basically said(which is 100% correct) is that your terrain ONLY looks good because you put some pretty models in it. I mean, for example - a cake wont be tasty if you color it fancy colors and add some fancy decorations and shape it in a special way. That wont make it tasty; only pretty. Basically, you put little to no effort in making this terrain - you just randomly put some pretty models. You could do the same with the High Res pack - it will still look pretty, but that's not terrain. I mean... Think of it that way, what would happen if you replace all your models with the basic WC3 counterpart of them? That would look extremely ugly, right? Well, if you make a terrain in which replacing all doodads with WC3 counterparts would actually look GOOD, then you could say you made a good terrain.
THAT IS in case you are doing terrains FOR SHOWCASE, to SHOW YOU CAN MAKE PRETTY TERRAINS. If it's for a PROJECT(MAP, CAMPAIGN, MOD), then you can just be sure that this project is not going to be approved on the Hive.