Epic fail.
-20-30% is a bad measurement. 30% of 5000 calories is a waste of protein; 30% of 900 calories (when dieting down) is too little. Per gram, per kilo or pound of lean body mass is much more accurate.
-Men and women require different amounts of protein for optimal growth
-Dieting down requires more protein to retain muscle comparatively to gaining muscle, and you will always lose some muscle when losing fat and gain some fat when gaining muscle. There is no magic, unless you are on some kind of anabolic, in which case it doesn't matter. You can however minimise the damage in either direction if you plan correctly.
-There are plenty of specific studies that have research optimal amounts of protein consumption for muscular development. They disagree in some very minor things but ultimately the upper limit where you hit diminishing returns for men is 3g/kg of protein; for the vast population though this is a waste as we are not training like elite athletes a more reasonable amount is around 2/2.3 per kg of LBM.
-Lean Body Mass (LBM) is not taken into consideration in the article. 30% protein for a 4000cal diet for a dude that weighs over 100kg and has 30% body fat is not optimal, as his lean body mass is only 70kg.
-How much protein can you absorb? Seriously guy. Ever heard of diurnal cycling? Your body adjusts.