Those kicks aren't slappy at all. You're mixing them up with the kind of kicks you'd see in semi contact point fighting. The definition of a slappy kick is one that doesn't really do any damage even if it lands cleanly. Slappy kicks can effectively be ignored in full contact fighting. Olympic TKD is full contact, and you definitely can't let the power kicks land cleanly without being dropped.
I spent over a decade in Karate, Kickboxing, and MT -- sort of a blend of them all, I guess Kickboxing is the best term for it. What really kills those spinning kicks is the low kick rules because they are easy to counter by angle stepping in to dodge and kicking out the support leg. This is how MT guys always dealt with it. It's pretty easy.
Without the leg kick rules though, these kicks become a lot more common. Watch the old long pants style American Kickboxing matches. They are wearing boxing gloves, hitting full contact, keeping their hands up in a boxing guard, and they are still getting knocked out left and right by these kicks.
A spinning kick like that is pretty much the most powerful kick you can throw as a human. If you've ever sparred a guy who likes spinning backfists, it's the same thing but on steroids. You don't want to stand still in the pocket and just take it, even with a hand up. Imagine that instead of a gloved hand coming around it's a bony, unprotected foot. And the power of the strike is way greater due to the longer lever of the leg plus much larger posterior chain muscles driving the acceleration.
Spinning kicks are risky and tough to connect with, which is why you don't see them as much. But if one connects, it's pretty brutal. Look at MMA to see the results when they (rarely) land cleanly. They do serious damage. They're not slappy at all.
