There are many circunstances, first of all, when in the building he was trying to kill them without making a noise, so he couldnt use his full power as it's difficult to fight in a small space without making noise, and secondly when he was outside he was runing away so it makes sense to be easier to defeat him, and the young master used one of his families main tecnics, so this is also showing us how much more powerful his family secret tecnics are when compared to other simple ones. Also this Knight was planted as a spy, and to be a goos spy he probably was weak when he joined so that he could fit in, so he grew as a knight in the family while being a spy, but he did hide some tecnics he learned along the way, but that was not enough against tecnics much more powerful. And lastly this fight is very important because it shows us that the young master isn't actually weaker then the MC right now, because when they fought at the academy, he couldnt use his families secret tecnics in front of all those people.
No, how did he overpowered by two genius kids when he was a right hand of such a powerful man thats capable to oneshot the golden golem when ronan can't even get a scratch on jt? Also he was so confident when facing that golden golem that you could say that he was strong when they saw it in the cave. If not how did he even become a right hand if he was that weak.