"Interesting" perhaps...but filled with some "wrongess" for sure...
Resampling anything anywhere will have an impact down the road when it comes time to sharpen...which by the way the author failed to address at all. (the failure to even address resampling for the contact sheets and the use of output sharpening for the printing makes this whole test nothing other than mildly "interesting").
The other main part of the equation that the author seems to have glossed over (since he may not understand it) is the issue of print SIZE and the human eye's visual acuity (loosely called "resolution").
It's not easy to compare the "PPI/DPI" of human vision to printed output but you can. Human visual acuity is generally defined as being capable of resolving a spacial design whose features are separated by one minute of arc (or 1/60th of a degree). What that one minute of arc represents is entirely dependent on viewing distance. The eye can see more resolvable detail closer than further away (assuming you don't bee reading glasses–all of this is based upon 20/20 vision).
Given that 1/60th is 0.00029089 radian, you can calculate the threshold of visual acuity for a given distance. Calculate the limit L of visual acuity at distance D with this formula L=D*TAN(0.00029089).
What this will give you is the following viewing distance and resolution required table:
Viewing Distance (inches)____ Limit (inches)______Resolution (DPI)
8 _______________________ 0.00232 ______________ 428
12 ______________________ 0.00349 ______________ 286
15 ______________________ 0.00436 ______________ 229
18 ______________________ 0.00524 ______________ 191
20 ______________________ 0.00582 ______________ 172
24 ______________________ 0.00698 ______________ 143
A couple of things related to the above...these are not my numbers but Bruce Fraser's numbers from his Real World Image Sharpening book )both the original and the one I updated). But since I'm coauthor of the current edition I stand by them (I was also the guy who asked Bruce the original question of "how many DPI can our eyes see Bruce?" which he referred to as my sending him down yet another rabbit hole).
The above assumes 20/20 vision...there are a lot of people whose vision is better or worse. The older one gets, the less close focus vision most people have (and the more people who need to wear "reading glasses"). So you milage may vary...
The above also assumes high contrast line pairs which while useful for measuring don't automatically translate directly to low contrast photographic textural detail. So, the odds are the practical limit may be a bit larger and hence the required resolution for continuous tone is prolly lower.
In general the "intended viewing distance" is said to be about the 1-1.2x the diagonal. So if you are holding an 8"x10" print in your hand, you'll prolly be holding it about 13-15 inches away from your face and be able to resolve about 286DPI. If you move it closer, you'll need more resolution as the eye can resolve more. If you move it away you need less. If your image is 30x40" the intended viewing distance is about 50" and you would need far less resolution for the print.
The downside about intended viewing distance is it assumes the viewer wants to see the entire image in their visual field...which of course is not always the case. Bruce used to say that the "intended viewing distance for a photographer was limited only by the length of their nose (or the magnifying power of their loupe)"
The next issue is the resolving capability of the medium. Even the smoothest matte papers can't resolve as much detail as glossy or semi-gloss surfaces. Beside the effect of dot "gain" you also have the issue of dot diffusion. So how much resolution you NEED for a continuous tone appearance in a print depends on the viewing distance and the nature of the media.
In general Bruce used to suggest 180PPI-480PPI depending on the viewing distance (either intended or likely) and the media.
The other question (which is of course close to my heart) is how the heck you sharpen an image for optimal printed output. That's a different subject that somebody could write a whole book on...oh, wait, we DID :~)
So,while the original author has some "interesting" ideas and suggests some "interesting" tests, I'm not at all sure his conclusions are as "interesting"...