Based on Harrison Clark's past experience, there's likely only one Spherical Battleship in the Orion Arm for the Compound-Eyed Observer but 999,999 Prism Ships.
So the theoretical probability of him encountering a Spherical Battleship is one in a million.
However, perhaps due to luck or the fact that he was already outside the Morrowind Empire's territory, the Blade Mantis that previously targeted him belonged to the Spherical Battleship.
Walking by the river often, the ship was bound to get its shoes wet eventually, especially since it had been patrolling outside for centuries.
It seems that the Spherical Battleship had already locked onto Harrison Clark, so it didn't enter warp or light-speed states, but rather steadily approached him at slightly below the speed of light and stopped suddenly at a distance of two million kilometers from him.