This is totally disingenuous. You want an exact legal definition of what an assault rifle is, and then you'll just come up with a gun that does the thing that the government is trying to prohibit in a slightly different way. That's exactly what the article is about. We prohibited automatic weapons, and so people come out with cranks and bump fire stocks that allow for automatic fire without violating the legal definition of more than one bullet per trigger pull.
I bet you know the history of assault weapons, but for those that don't, it makes this all understandable. In WWII, Germany's infantry primarily carried Mauser rifles (https://en.wikipedia.org/wiki/Karabiner_98k) These were bolt action, meaning you had to pull the bolt back and forth between each shot. It shoots a high powered round, and it has a reasonably long barrel, which means that it can be accurate and deadly from a few hundred meters, but it also means that it has a fair bit of recoil. It has a ten round magazine, which is somewhat cumbersome to reload. This type of rifle works fine for hunting, and for firing from one trench to another or across an open field.
When Germany found itself doing a lot of house to house fighting with the Russians, this was not a suitable weapon. These firefights tended to be over very quickly and at very close range. There wasn't time to aim, and often soldiers shot from the hip while advancing. What was needed was a weapon that was fully automatic, handy, with a short barrel, fireable from the hip, with a large ammunition capacity and quick to reload with not too much recoil. The result was this: (https://en.wikipedia.org/wiki/MP_40), a submachine gun.
Later, the Germans realized that although the MP40 was great for house to house fighting, for longer range engagements, it wasn't accurate enough for longer ranged engagements. They needed a compromise weapon: The StG 44 (Assault Rifle 44). The StG 44 had an intermediate power round and a medium length barrel to not create too much recoil, and be easy enough to handle close in, but have enough power and accuracy to fight at medium distances. It could fire either in semiautomatic or fully automatic mode, fully automatic for a close assault, and semi for long range accurate shooting. It had a pistol grip so that it could be fired from the hip, and an easily changeable 30 round box magazine, so that it could be reloaded quickly in a fast firefight. The AR-15/M16, AK47, and virtually all general purpose military rifles from the second half of the 20th century follow this basic concept.
This is all to say, that the basic design concepts of the assault rifle, intermediate power cartridge, selectable semi/fully auto fire, medium length barrel, pistol grip, and detachable, high capacity box magazine, make this a weapon designed for war. You can argue until you're blue in the face that none of these "evil features" make it any different than a bolt action hunting rifle, but they all exist to make it more suitable as a weapon of war, and you know it.
The very same features that make it useful for soldiers to kill enemy soldiers make it more lethal in the hands of a mass shooter. The pistol grip makes it easier to shoot on the move or from the hip. The high capacity box magazine gives a lot of firepower without reloading, and makes reloading fast. The less than full power cartridge makes rapid fire (semi or full auto) more manageable, and the medium length barrel makes it accurate at medium distance while not being unwieldy in close quarters. You can't tell me that what happened Sunday, or what happened in any of the recent mass shootings would have been any where near as deadly if the shooter had showed up with a Remington 700.