I've already addressed the differences in 'preppers' and the definition. You will note, also, the author of the OP article didn't quite break it down, either.
Using both ends of the spectrum, let's try this...
1) Doomsday preppers... thinking the EOTWASKI is coming, preparing for the time they will have to rely on what they have on hand... including a firearm. Why would a weapon system that is almost infinitely adaptable (simplicity of use,) that is easily maintained (mechanical simplicity,) that is reliable (superior reliability,) and as a bonus, easy to fix if something goes wrong (new parts) not have an advantage?
2) Pedestrian preppers... preparing for 'adverse and tumultuous times'... and having a firearm available, but thinking they will not be required to live in an armed environment. Why would a weapon system that is almost infinitely adaptable (simplicity of use,) that can be easily maintained (mechanical simplicity,) that is reliable (superior reliability,) and, as a bonus, easy to fix if something goes wrong (new parts) if not by that person, but by another person with reasonable knowledge of the platform... not have an advantage? That is setting aside the author's nonsensical claims about fragility and unadaptability.
The AR-15 IS a good choice for an average non-gun owner. Although the controls may or may not be ideally laid out for any one person, an AR is easy to operate, has good visual cues (safety selector,) and is certainly easy to maintain even at a basic level. I can think of a bunch of other platforms that do not meet that requirement, including the AR-10, and others that are not even close. Conversely, if you wanted to break it down to simplicity and ease of use, with no spare parts... you might as well go with a single-shot break-action rifle, but that is not the subject of your OP.