JJMack, please read that sentence you quoted more carefully: I never stated Photoshop does any pixel interpolation. But it does interpret the size of any placed file.
Unfortunately, your test setup is just about the worst example setup, because you are placing two files with a LOWER ppi than the 500ppi for the main document. In that case Photoshop will automatically scale down the images to fit the document, because their scale would be far greater than the document you are importing into - and they will indeed all end up exactly the same size.
Here is my example. I set up a letter sized document at 300ppi. Images placed have their ppi's captioned. The first two images on the left are imported at the actual true 1-1 pixel scale (both set to a 300ppi setting, just like the master document). Those are placed one-on-one pixel. The others are either scaled down or up, depending on the ppi set.
THAT is what the OP is complaining about. Importing assets with various ppi setttings leads to widely varying scale interpretations by Photoshop. Notice the blurry up-scaled owl on the right (I included the true pixel resolution cut-outs that demonstrate how it displays on the screen at 100% view).
Now, that makes absolutely sense when dealing with print (InDesign and Illustrator behave in an identical fashion). And sometimes with web (retina, but even then one has to make certain the ppi is always set to twice as high as the master doc file, e.g. 72ppi/144ppi - any other ppi value will lead to unexpected/unwanted results).
I also would like to argue for an option to ignore the placed image's ppi parameter, and just be able to place bitmap files 1-1 in terms of pixels. Because smaller graphics (like the owl icon above) just might be saved with a wonky ppi - leading to crazy scaling, and very blurry graphics. And then we are left with trial and error to figure out how to scale down that layer to true 1-1 pixel interpretation. Because there is no transform option available in Photoshop that says "scale to actual pixels".
Since I am aware of ppi and how it works in PS I know how to solve this, but for most users ppi remains somewhat obscure as a concept - especially for those working primarily for web and screen jobs. So a checkbox to ignore ppi whilst placing a bitmap could simplify life. Even for people like the OP (and me) who are acutely aware of ppi, but merely wish to work with the actual pixels during import and avoid any potential up-scaling due to funky ppi file settings, it will save time, and make the workflow more efficient.
Also, as far as I am aware, there is no option in Photoshop to set a layer to a custom ppi setting easily with the transform tools. At the very least include a "scale to actual pixels" option. Do not make things more complicated than they need to be.
Clear now, I hope?