OB,
I tend to agree with your assessement of the penetration. I have some limited experience with the impregnated wood stuff. We used some of this for making model airplane props for a formula 1 race plane. I believe the impregnated wood we used was for electrical insulation applications. Also, Browning dabbled with similar impregnated wood for the Challenger pistol grips.

I'm suspicious that the best of the 'in-the-wood' finishing techniques may only penetrate the finish solids to a depth measured in thousandths of an inch, similar to casehardening. Sectioning a sample block that has been finished in your favorite method will tell, I suppose.