Of course, Lu Qianshan knew that the Laines Empire's fleet was already on its way, and he not only knew, but also knew the specific location, and even roughly how many ships there were.
Yes, roughly how many ships... Because the scale was too large and the organization too complex, nobody could clearly state exactly how many ships the Laines Empire fleet had.
Ships occasionally fell behind, as some transport ships were originally very slow, and the closer they got to the destination, the more likely some would lag.
Some merchant fleets even deliberately fell behind: they themselves did not want any conflict with the Great Tang Empire, as that would seriously affect subsequent commercial activities.
War, after all, will eventually end. Even if the Great Tang Empire loses Dragon Island, it will likely continue to exist. When doing business with the Great Tang Empire, who knows if they will hold one's past actions against them?
