It seems he's saying - don't do both (i.e. capture and output) He provides side by side examples.
It would be useful if Michael Rather had half a clue about what he was talking about. Yes, if you OVER SHARPEN in Develop and then use HIGH for the output sharpening you certainly can end up with an over sharpened results...DOH. Misusing ANY tool in Lightroom can screw up your image, not just sharpening.
What this alleged "expert" seems to fail to grasp is HOW to use the capture sharpening and output sharpening properly. You don't do a side by side comparison at 1:1 to make any useful determinations...you need to actually MAKE A FRIGGIN' PRINT. (output sharpening is designed not to look at on screen but after you make a print. That's the ONLY reliable method of judging the results)
I suppose I could point out Bruce Fraser's original article outlining the concept of a sharpening workflow but I'm not convinced that Michael Rather would understand the concepts. Suffice to say that Bruce's concepts are what's embodied now in Lightroom. And if Michael Rather is trying to do video tutorials to teach OTHER people how to use Lightroom, it would behoove him snuck up on getting a clue. Cause, at the moment, nothing in this video indicates that he does.
Bottom line? Seriously, don't pay any attention to the man behind _THAT_ curtain...(he really, really doesn't understand what he's talking about.
Ah heck, ok...here's Bruce's original article...
Thoughts on a Sharpening Worlflow by Bruce Fraser