Did you even look at the URL in the OP? It shows several phones that are permanently bent - some plastic, some metal. It shows plastic phones, like the Galaxy, with a cracked display from where it was bent, plastic phones that are permanently bent (BlackBerry Q10, Oppo) as well as other phones with metal frames like the Sony Xperia Z1 and HTC EVO. It also shows various other older models of iPhones that are bent.
No phone is immune to this, and just because it's plastic and kind of "bends back" does not mean the screen or plastic won't crack, etc.
I'll tell you exactly what this is about. Millions of existing iPhone users now have a larger phone in their pocket, and because the previous models were smaller, they were just under the bending threshold (due to the weight of the person, size of pockets, whatever) and they didn't have a problem. Now with the larger phones there is more leverage to exert more force (plus being thinner might make them weaker as well), and suddenly the bigger phones can't handle the stresses that the smaller phones could handle. If these people were to stick a Samsung S5 in their back pocket bad things would happen too (and it just so happens that the older, smaller iPhones were tough enough to handle that).
Is the iPhone 6 as tough as the smaller previous generations of iPhone? Almost certainly not. Is it as tough as other phones the same size like the Samsung Galaxy? Probably so.