I wish Book would've detailed what's wrong with his game, without specifics it seems like he's just hating. I think Riddle needs to improve his psychology, selling and pacing, develop a character and learn to cut a promo, but I'm not a 2 time HOFer so it doesn't mean much when I say it.
Matt Riddle might not do flips but he's still a spot monkey, he can chain together a bunch of cool looking moves but there's no story there. To be fair, the same could be said about every lauded workrate guy today, that's the style Indy fans want to see and he's good at it. If Book went in on Riddle he'd have to go in on everybody's favorite guys and that would turn the IWC against him.
Booker didn't give specifics because it's obvious. Booker comes from a time where you actually had to know how to work, when he sees Riddle he just sees a spot monkey who doesn't know what he's doing. Most of these guys nowadays who are praised as good workers are actually pretty awful.