There is a guy (who you've linked to) who appears to be implying that shaft stiffness is a major component of shaft deflection (lots of people say lots of things, including that the earth is flat). Dr. Dave (among a ton of others of the most intelligent among us) says that the shaft stiffness is of relatively little significance to shaft deflection. Which to believe?
Well aside from their track records (one has an unbelievable and substantial record of being right, don't know about the other guy), and their qualifications (one guy has a doctorate in and was a professor of engineering with a strong background in mathematics and is obviously and proven through time exceptionally intelligent, no idea about the other guy's background or intelligence), aside from those things we actually have something much better in this case that can be used to determine which one is right. We are able to do tests for ourselves to find out with significantly more confidence, and Dr. Dave has outlined the ways that it can be objectively tested. Have you done those tests? If so, please report your methods and results. If not, probably time to do that as there is no sense in relying on theory and speculation and intuition and perception when there exists an easy way to find out with significantly more certainty.