Nothing that I’m saying is contradicted by that Wikipedia article.

I have, on my desk, no less than three 35mm film SLRs that will not allow you to see with both eyes open using a 50mm lens. I have already given you a link to an article explaining it, as well as explained it myself.

The image you see through a normal film SLR is the image that the lens is projecting onto a surface that is further transformed in the prism, you can see this surface by removing the lens and looking at the top of the inside of the camera above the mirror directly behind the lens. That image on the ground glass surface is then transformed using another set of lenses and mirrors in the prism so that you can put your eye to the lens and see it right side up, and focus your eye as if the image were not less than an inch away on a piece of ground glass.

There is no SLR on earth that does not have additional optics between you and the image projected on the ground glass. In modern cameras the ground glass and additional optics are a single piece with the flat side facing down and either a fresnel lens or a normal glass lens on the top.

Those optics inside the prism, that every single eye level finder SLR has, are what decide whether or not a 50mm lens shows an image to the photographer that is comparable in size to what they see with their other eye. If it is 1x magnification at 50mm it is the same size. Otherwise it is not. You can look up the magnification for any SLR. There is also the completely different coverage spec that SLRs have that tells you what percentage of the full image to be projected on the film that will be shown in the finder. You can have cameras that show the full image at lower magnification, in the same way that you can see the full image after printing on a 4x6 photo, or on a 8x12 photo.

What is crucial to understand, that you have continually missed, is that there is not a “neutral” spot that occurs naturally at 50mm. It is an artifact of design on many, but by no means all, cameras. A Nikon D850 has viewfinder coverage of 100% and a magnification of .75x at 50mm. That means that the viewfinder, with a 50mm lens attached, will show the entire image to be recorded, and the image will be 75% the size that my other eye sees it. It will give you a headache to try and shoot both eyes open. My Nikon F90x has similar specs for the viewfinder, to preempt any notion that this is because of digital. It is referenced to 50mm because it has to be referenced to something and the most popular focal length is the one that manufacturers settled on. Some SLR cameras show a smaller, but still complete version of the image that the lens is projecting. ALL SLRs need an additional lens in the prism to make it possible to see anything at such short distances. The nature of that internal lens and the prism is what determines the magnification not the lens that you attach.

If you go look at the Wikipedia link that you found there is a cutaway diagram showing the additional lens that you are viewing the image through. That combined with the article I linked earlier explain it very thoroughly.

Good luck with your journey to understanding of this concept.