FF Lens on APS-C body - confusing

I'm shooting with a D7100 (until my D500 gets here). I switched from the 70-300 kit lens to a 80-400 "pro" lens.  I loved the difference the 80-400 made.

So I'm listening to Tony Northrup's video, "20 things photographers get wrong," and Tony argues that since my "APS-C body only covers about 45% of the the lens, it is magnifying the defects in the lens by 200%" and I'm actually getting worse images than I would from the much cheaper kit lens.

Admittedly, I'm not photography tech savvy, but "in my mind, and to my eye," my pro lens shots look better than my kit lens shots.

Am I duping myself by buying good glass? Or should I strive for cheap glass for better images as Tony argues?