She has some interesting insights but I think she held back on passing judgement on the 'validity' of these mannerisms, I guess out of professional respect.
For me there's a big distinction between mannerisms/gesturing/conducting and physiologically required reactions fromthe body. For example the 'tongue-at-tip-of-teeth' effect on the facial muscles, the 'eyes-peeled-wide-open' are physiologically forced at moments where you're really reaching for a super exact level of dynamic or color where you still have some milisecond(s) window to absorb the physical feedback from the escapement and react. I don't want our great pianists don't waste hundreds of hours fighting the body to get rid of that, I'd rather they use that time better.
Or the 'looking around' is because the head for its use from its weight is optimal when not pointed forward in a particular moment. I think the perception is there's never a reason for the head to not be looking forward, but that's not always so so I would've liked her thoughts on that.
Or if you have to raise the shoulders to the point the neck tendons get tauter that coincides with a 'mannerism'-esque effect on the facial muscles that's anatomically unavoidable. I don't think these are real mannerisms.
There may be a grey area with people visibly counting the beat or singing a la Kissin where maybe they're semi-mannerisms, you get it because they grew up doing it and if it's not actually audible it's fine.
I don't buy the 'polyphony' from Gould or Jarrett talking or scatting, that has always struck me as someone trying to sound intelligent 'ah he's adding another voice to Bach', The counterargument is simply that no one wants to hear your added voice and they could've actually worked to stamp that out and simply retained their greatness while leaving it out, but hey it doesn't annoy me that much and it does confer some mystique to them and therefore the piano overall. I think there's enough room for exactly 1 pianist in classical and 1 pianist in jazz for this idiosyncrasy but probably not dozens of people trying to do that.
The literal acting and conducting is really absurd, this is just outright pandering to appeal to the widest audience possible.