You are comparing apples and oranges. Plus, you missed the point I made that the term "80%" has no meaning other than as an industry marketing term. ATF has no standard based on percentage of work required to finish a receiver. They have a standard, if one that can be considered arbitrary and malleable. That standard is that it is a gun, or it is not a gun, and what distinguishes that is quite different from one firearm design to another. In the case of the 1919 or M2/M3, mere drilling of rivet holes is considered so simple that anyone with a power drill or drill press can finish it quickly. On the other hand, imagine a plate where all the holes are present, but there is no charging handle slot and the profile is a perfect rectangle. You have a hand power drill and/or a drill press. How long is that going to take you to carve out the slot and the feedway, not to mention a couple of other features? Few home builders have the equipment and ability to do that. Of course, there is the KMP offering that is marketed as a "70%" plate, which has those features milled most of the way through the plate, leaving .030" or so to cut through. But it is still a rectangular plate with no features on one side, and I believe no holes drilled through. ATF has said that is not a gun, because it does require time consuming work to finish the feedway and the charging handle slot, in addition to drilling and chamfering all the holes. To them, that's good enough.
Now some might argue that it isn't a gun until the box gets riveted together. I might argue that makes perfect sense. However, when your side plate is finished, a couple of hours in your garage and you can have a functioning gun assembled. That's the ATF view. If it's that easy to complete the manufacturing process, then the state of the receiver plate is where the rubber hits the road. If anyone want's to collect a few hundred thousand dollars, maybe a million, and sue ATF over this standard, please go for it. Otherwise, they get to enforce their standard.
An 80% AR receiver is a different animal. You need a mill, or you need one of those fixtures and a some time just to get the receiver finished to usable state. I have not done those, and don't have a feel for the time and tooling. But I am pretty sure it's far more complicated than drilling an old 80% Browning plate. To sum it up, just because both Browning RSPs and AR 15 lowers have been marketed as "80% receivers," it doesn't mean the same standard is applicable.