andrusoid

Watch the beginning of the video carefully. That flash is the display arcing to ground. Hear the loud buzzing sound? That's not special efx. If you look at the lower right of the iMac, you can see that it is still plugged into AC power! So, yeah, a short to ground could blow the PSU, and motherboard. (Also not helpful was the vertical position, no esd mat or grounding...) 

I'm a certified Mac technician and no AASP would sell the parts to me either.

The "cheap zinc screws" that Apple uses are meant to deform slightly to keep them from loosening and falling out. That's why technicians are directed to use new screws in several different repairs. There is at least a very fair chance that the screw was over-torqued at least once causing it to break. The screw threads actually stretch the core of the screw until it snaps. I get that most folk would not know about the one use prohibition, but I can't say that it is fair to blow this incident up to the "flaw in the Vesa mount" level. 

Fortunately, this is AppleInsider, and some good reporting included some reasons for using softer metal screws were given. Thank you insider types!