As a wildlife photographer I've always looked at PPB - pixels per bird. If a camera is going to give me a higher PPB count then that beats a camera that will give me the same PPB count but smaller pixels. I alternate between the R5 and R7, and they both give me great photos. Any knits I can pick with the R7 have to do with performance and not price-performance. Were Canon to give me a camera that's powered like an R5 with a sensor that performs like the R5 and the size of the R7, and the R7's focus system, I'd be all over it, even knowing I'd be paying over $3K for it.
My biggest issue with cropped sensors is that camera companies perpetuate the stereotypes. I believe Canon could have given us the camera I describe above, but because of existing perceptions the idea of a $3K cropped sensor camera fails in the board room, not in the field. Before coming to Canon I shot a pair of Nikon D500's. Amazing camera and their top of the line cropped sensor. Yet while it was used widely professionally for sports and wildlife, Nikon itself categorized it as a "consumer" body, making me ineligible for NPS by using them.
Having said that, my take on this guy's video is that I know what he's trying to say but he does a lousy job getting to the point - mainly because it's impossible to make a point when there are no apples to apples comparisons. And by that I mean identical tech, identical MP's, cropped vs. full. Were that possible then science dictates that full frame wins every time on everything but PBB. Because a bigger pixel with all other things being equal is going to to provide better results. And with identical tech and identical PBB you're effectively dealing with anywhere from a 60 to 70% larger sensor on the full frame, so it means your cropped camera is likely priced out of the market because you have more flexibility with the full frame.
This is a "which hammer is better" argument. If you don't know hammers and don't know where and how you're going to use it, which one you're going to buy as your first likely depends a lot on whether or not you hang out with roofers or framers and if you're not careful you can walk out with a 20 oz. claw to drive brads finish nails into that quarter round at the baseboard.
Horses for courses. And I haven't even started into what you stick in front of it.