Artur was truly shocked by the strength he felt from Robert during their brief fight. They didn't use any mana at all and relied solely on their physical strength and swordsmanship skills.
In that single exchange, Artur could sense that Robert possessed far greater power than himself. If they were to fight seriously, Artur would be defeated in just a few moves. This astonished him because Artur believed he had grown much stronger than before. With the resources of House of Valya at his disposal, Artur thought his strength wouldn't be so far behind Robert's. However, Robert seemed like an insurmountable mountain, leaving Artur feeling almost hopeless after just one exchange.
Artur didn't think Robert had reached Level 3 Constellation Mage, so the power disparity he felt was purely due to Robert being physically stronger. It made him wonder what kind of food the Gaht Family provided to create a monster like him.