...I think Artie Morris is in an excellent position to provide insights and valuable comments - but his position as a salesman of Canon products means we should treat what he says with some degree of caution.
It's important to keep such motivations in mind. For example, his recommendation of the 70-200/2.8L IS II + 2xIII over the 100-400L...it doesn't make much sense from an optical, AF, or handling standpoint…but it makes perfect sense from the standpoint of financial gain for Artie Morris.
There are plenty of reasons to recommend the 70-200+2x combo over the 100-400L, and making an extra buck off Canon isn't the best, not even remotely. For one, it's more versatile, especially in poorer light (you can pop off the TC and creep up closer, if needed, with an f/2.8 lens instead of being stuck at, AT BEST, f/4.5 @ 100mm).
There is also the whole design aspect. A lot of people, and I would even go so far as to say a majority of people, don't like the 100-400L push/pull design. I personally like it, but it is an oddity overall. I believe lot of people prefer the classic dual ring design, one to focus one to zoom. That alone is probably more than enough reason for Art to recommend the 70-200+2x combo over the 100-400L.
There is minimal to no loss in IQ between the two options. The 70-200 f/2.8 L IS II is a newer lens design, with better IS functional up to -4 stops, vs. the 100-400mm design IS which is at best functional at -2 stops. The MFD of the 70-200 is much closer (47.2" vs. 70.9") than the 100-400mm.
The 70-200mm focal length, especially with the fast max aperture, also makes it a much more versatile general purpose lens than the 100-400mm. You don't see many wedding photographers using the 100-400 (if any), however the 70-200's are a staple. The 70-200 with TCs can nicely round off a kit that is comprised of a fairly minimal set of lenses. Many photographers could get away with nothing other than the 16-35mm, 24-70mm & 70-200mm, or maybe some alternative with a couple wide primes.
The only real major drawback of the 70-200 f/2.8 L II with TCs vs. the 100-400mm is weight. The latter is quite a bit lighter weight than the former (without the TC). Overall, however, the 70-200 is a vastly superior lens. I see absolutely no reason why Art would recommend the 70-200 just to pad his own pockets. He doesn't strike me as that kind of individual...and his reputation is probably one of the most valuable things he has. I don't see him destroying that to make an extra buck from Canon by pushing a more expensive lens. I don't think expense has anything to do with it...I think the simple fact that the 70-200/2.8 II is a better lens period, even with the 2x TC, is the reason he pushes it.