First, this OP does not demonstrate any attempt to test these devices scientifically. There is no way to know if the same amount of pressure is being applied to each device without a way to measure it.
Second, the OP is not pressing in the center on the iPhone 6 or 6+. One thumb is under the volume buttons (not in the center) and the other is under the center.
Third, the phones (all of them) are more likely to bend if the pressure is not exactly dead center. By pressing dead center, there is less area for flex and pressure on the center than if the pressure were applied off-center.
Fourth, the design of each phone may reveal different stress points (i.e., weak points) based upon each individual structure (camera in the center of the Nokia Lumia).
Fifth, (lack of) evidence of a bend does not mean the internal components of the phone weren't damaged.
Sixth, all metal phones will bend with enough pressure.ff
Last, I do not doubt that the iPhone 6 (and especially the 6+) will bend and/or twist; however, this video is worthless. What is needed is a way to test the iPhone in a manner that replicates the pressure exerted on the phone when in a pocket.
Apple has a problem on their hands--the question is whether it is a design flaw or a manufacturing flaw.