I do know, I grew up around these types, heard the laws and politics a millions times, seen it all first hand.
Your article is more misleading than wrong. As it says here..
One law in 1935 all but banned automatic weapons like the Tommy gun. And another in 1986 prohibited fully automatic weapons, except for pre-existing weapons that were grandfathered in
Just like I said, two relevant laws, the big one being the 1986 gun control act just as I remembered. The way in which they are misleading is there are many thousands grandfathered in from back then, and you can buy a modern ar-15 built today, put in an autosear component and maybe a couple other things defined by the law as being what parts of the gun make it a gun, built in the early 80s, and presto, new legal machine gun.. Mostly new anyway. New enough that there is little danger of the existing number deminnishing to non existence any time soon.
There is a whole market for class 3s. Expensive, but thousands expensive not hundreds of thousands expensive. Within reach of the middle class wealthy and above so inclined.
There is another legal trick too, it's also legal to get an FFL and manufacturing permit and build your own automatic from scratch, not too hard for someone with basic machining know how. And what is defined as a legitimate gun manufacturer can be a little dubious. Is a guy with a garage a gun manufacturer? Sure, so long as he has an FFL and a permit from the ATF.
Trust me man, I was around gun nuts my whole adolescent life, I know the rules, the broad version anyway I might have forgotten some of the details. The point is there are many Americans out there with legal automatic weapons that look pretty modern.
The article is a little misleading with how illegal "basically illegal" is. Probably not on purpose, they probably weren't sure themselves. Trust me if you'd seen what I've seen you wouldn't be thinking they were "basically" illegal
Anyways, you're welcome to research it more if you like if you still find it hard to believe, I wouldn't blame you. You wouldn't think they are legal. And the legal situation is sufficient to keep any amount of crime being associated with them which combined with the mistaken belief that they are unattainable is why nobody has done anything further since 86. It is completely irrelivent as far as high profile mass shootings go, all are with much cheaper and easily obtained semi autos, none have involved any class 3s.
A similar situation exists with silencers (or suppressors being the more correct word in the minds of most gun nuts). They are also widely believed to be illegal but can still be attained with a class 3. But you're right, it's off topic so I'm happy to end my part of this side discussion here.