I did have to look him up, because honestly I've never even heard his name mentioned on talkbass. Certainly I've never seen his name on any list of "world's top fretless bassists." But I did visit his website, and enjoyed the music. He seems like a regionally successful, multi-talented player/songwriter/composer/producer. Which raises the question: How can such a talented musician have such awkward technique on the fretless? Here are my thoughts:
1. Maybe that's not his hand in the photos. Not every bare butt you see on screen is the movie star's; sometimes it is a body double. When I worked in publishing, there were definitely moments like, "hey, we have a blank half page in the layout; let's snap a photo of someone's hand." Not claiming it is stunt hands in these photos, but it is certainly a possibility.
2. I've written an instructional book, and let me tell you, there is a strong tempation (and often pressure from the publisher) to include certain "correct" or "advanced" techniques, even if that is not actually the way I myself play out on a gig. For example, I am not generally a "one finger per fret" player, but I tabbed it out that way in my boom, because I felt it was what I was "supposed to do."
3. The book was published in 1983. That was the height of the Jaco era when fretless bass was just bursting into the scene. Maybe Clive Harrison's fretless technique has evolved some in the past 34 years, and he would give different lessons if you were his student today?
4. Sometimes great musicians have less than great technique.
Anyway, if you want to get good at fretless, my advice is to get a good teacher, and study ALL the great players (Jaco, Manring, Piano, etc.) not just one photo in a book from 1983. Take the best from each player and combine it all to form your own personal style.