Yeah, but you'd also think he'd stop after the first match and acknowledge his misjudgement
The spin move is wild, I guess it only works for him because it will always work at least once as he catches his opponent off guard and due to his strength just blasting through any possible defense.
I think there are a couple of other details that work in his favor.
The biggest is the fact that he sets up the move by first slashing at his opponent’s leg to cause them to get off-balance. Beryl even notes that by doing that it makes it difficult for the opponent to be able to counter-attack in time.
He then continues on following through with that slash by jumping up and adding even more energy to the next attack. He also uses his free hand and arm to help block the sight of exactly where he is going to finish his attack to make it just a tiny bit harder to block.
And he’s incredibly (nearly inhumanly) strong, to the point that he’s even noted by Kuruni as having broken the arms of some of the other knights during the sparring sessions when they try to block him.
However, we do see that it’s a risky move when Kuruni uses it against one of those Holy Knights later on in the story.
During that fight she uses the same spinning attack, and the Holy Knight (who is using a rapier meant for stabbing moves) is able to hit her on her back. But, whether intentionally or accidentally, the rapier gets deflected by the sword sheath on her back.
Also, while I doubt that this particular tactic would work well in actual combat, it should be noted that there ARE a couple of IRL moves that involved spinning in a circle.