Actually that's more of a case of people misunderstanding their advice/writing. Those links are simply stating that if your form suffers from trying to do a plank for a prolonged period, you're doing it wrong and should concentrate on shorter times while focusing on doing it correctly/contracting to build a core without risking injury to yourself. If you can keep proper form and tighten your abdomen, glutes, etc.. then holding it longer will only strengthen you overtime. You only risk injury if you are incorrectly doing it like most fats. Fats will also interpret that article as maybe I "should just only do 10 second intervals and never look at improving my times because candishuns."