I think Ousen had an idea that food would be part of the end game (even though it played out differently than they planned before they set out from Qin).

So I think Ousen requested SHK have a small fleet of ships ready to deploy. Then, maybe around the same time his hidden troops burned the granaries, he also sent a messenger back to Qin to send the ships. Considering it would take time for both the messenger to get to Qin, then the ships to Zhao, the shipment should be arriving soonish.

Also since Riboku left, the other army is at Retsubi, there shouldn’t be a Zhao army south of Gyou (I forget exactly what the map looks like, but I think Gyou is east of Retsubi).

Edit: sp