Rowan was not that invested in the battle, he could already infer who was going to win it. The Celestial Creator may be powerful, but it was obvious to him that he was not here with his true body, and this was just a projection. No matter how powerful he was, things had changed with Vyraak that he could no longer equal what this dragon had become.
The weapon he had given Vyraak was a Proto-Source Level Treasure and was as close to a Source Level Treasure as Rowan could ever make it.
This blade was amongst the most potent killing treasures he had ever created, which, for Rowan, was saying something. In addition to all of that, there was the fact that he had imbued the weapon with his Weapon Master's trait in its blooming consciousness, making whoever wielded the blade have the possibility of matching Rowan's martial prowess in the field of weapons.