Dragon Island was not without spies from other empires, in fact, the number of these intelligence operatives was not insignificant. After all, as one of Tang Country's secret weapon development bases, it was certainly a place other countries took special care to watch.
Unfortunately, the spies from these distant countries mostly could only reside honestly in Weigang, which was still some distance from the truly important Dragon Harbor.
To get to Dragon Harbor, they had to cross the entire Dragon Island, something that not just anyone could accomplish. They would have to pass through forests and cross the blockade lines of the Tang Country forces, not to mention sneaking past a bunch of disorderly factories that could well be called secret bases.