Perhaps the genuine incompetence – in publishing nonsense dressed as “science” – is limited to some of MDPI’s softer “journals” like Nutrients: http://www.australianparadox.com/pdf/RRsubmission2inquiry.pdf
In any case, readers and scientists shouldn’t trust a publishing entity that refuses to correct obvious errors/misrepresentations and false scientific “findings” self-published by highly conflicted scientists, “researchers” who really are just excessively confident Glycemic Index advocates.