According to the usual behavior of the Pingyang Patrol Army, their decision to show up at this time clearly wasn't likely to be for a fair handling of this conflict.
Given the commotion that Gu Ran had made upon his arrival, it was impossible that the other party hadn't noticed. However, no one came out to stop them even when the two were officially engaged in combat, which meant that the patrol army of the Blaze Prairie defense area did not intend to intervene.
In that case, given their past behavior, they would probably only appear after Gu Ran, the party involved, had left the scene. Their hasty arrival now was almost immediately after the discovery of Zhou Guoli's death, and the implication of coming to seek accountability was unmistakable.