The reinforcement that came to Aurebor was only the mobile fortress. The main army at Hydrurond was moving at a slow march toward Liguritudum. It was difficult to mask the movement of such a large army. Hence, John had the main army continue heading to Liguritdum.
He intended to attract Liguritudum's attention. With Liguritudum believing the combined force of Themisphere, Sangrod, Palgrost, and Hydrurond was heading in their direction, it would be unlikely for them to send reinforcement to Aurebor.
The second reason was that the joined army had a slow marching speed. Some of the soldiers still rode common steeds. Additionally, they needed to build a supply line while marching into Liguritudum. They couldn't move fast.
The mobile fortress, however, could. With the upgrade of its movement speed, it could travel at a speed equal to a rare-grade steed.