https://www.theatlantic.com/magazine/archive/2012/04/the-man-who-broke-atlantic-city/308900/
Didn't really read it thoroughly, but the article doesn't seem to be lying with the fact that he beat those casinos with pure skill since the first casino he visited already had security cams analyzing his every movement to prevent any type of cheating. Card counting may be done in the mind, but it still needs obvious eye movement as well as some kind of pattern or rhythm that may be helpful to you in remembering (imo that is). That, or you can just have a perfect memory, which is almost impossible.
Although it seems like the biggest factor here was removing the edge for the house, and instead, giving it to the player. I've read some other articles, and they've also detailed about them winning on casinos not only based on skill, but also by negotiating with them on discounts and the like, they then start to break the bank of the casinos.