The newly arrived ships were the Liguritudum ships that had been captured and reintegrated into the Themisphere and Palgrost's fleets. The allied fleets had arrived nearby but they stayed back and only had the captured Liguritudum ships sail forward.
Aside from interrogating Gridhacker about Jeanny, John also interrogated Gridhacker about the blockade in this border river. He was sure the enemies left some ships behind to harass the allied armies from crossing the border river.
The first thing John asked Gridhacker was if there were any players following those ships who guarded the river. If there were, the news about the Liguritudum Armada's defeat would surely have reached those ships. If there were none, then their communication could only be through a transmission device.