Fan Lou put on an exaggerated show of realization and gave a reason that anyone would find ridiculous, as though he thought Feng Qing was a moron.
However, Feng Qing had no choice but to believe Fan Lou because he had already aligned his story with the Dark Pufferfish. Later, they would find a way to relay the same reason to Feng Qing.
Feng Qing was still suspicious of him now, but once he heard the same story from the Dark Pufferfish, his suspicion would turn into deep trust.
Now that Fan Lou had become the bridge between the Dark Pufferfish and Bloodians, he could exit from Feng Qing's protection. However, he chose not to do so because he knew that he would only be able to freely tap into the Chaos Wind Demon Wolves' power by gaining Feng Qing's trust and staying by his side.