It's one man's opinion, based on "6 years of turtle experience", his turtle appear healthy, so technically it's do able. Sounds like he's permanently dry docking them and only putting them in water to feed and re-hydrate.
I personally don't think it is ideal. By his method you, and not the turtle, are the one regulating their hydration level, body temperature, and need to feel secure. These are aquatic turtles and by nature they spend the majority of their time in water except for when they need to bask. Keeping them with with access to water and dry land, the turtle can decided what it needs to warm-up/cool down/drink/hide/sleep and not rely on guesswork by the owner.
That's why I asked is it normal because to me it seemed little bit strange. Fortunately my turtle lives in tank and top of the tank she has ATBA, all the necessary lighting UVA/UVB and heat bulb. In future (this summer) there will be bigger and better tank and ATBA.
Thank you for your response
"When an animal puts their trust in us, may we never let them down..." -April Peerless-