That's just doublespeak to cloud the issue of KDE not being lightweight... He mentions all the right factors himself: low memory, low cpu use, fast, not too many features. His complaint seems to be that there is no definition of a cutoff point from where something ceases to be lightweight. That's like saying you can't call an airplane fast transportation, because there is no fixed definition of 'fast'. However, most people who say an airplane is fast are comparing it to cars, walking, boats, etc. They are not referring to comparisons with the speed of light. And, lets be honest here, if a desktop environment is being called lightweight, that means compared to KDE and Gnome. Don't tell me you can not be more memory, CPU, feature and speed efficient than those two.
Disclaimer: I love KDE, use it every day and I happily trade my resources in for the features I get for it.