After learning the truth, why did Dev believe he couldn't have possibly recognized Baba as the Sanyasi? What was his reasoning?

Dev's failure to recognize Baba was rooted in a prior encounter which created a "veil of Maya" for him. As he reflected in Chapter 40, the Sanyasi had first visited him before the Udyapan invitation was even sent to Baba in Shirdi. On that initial visit, the Sanyasi was collecting funds for cow protection. Because this meeting had a different, worldly purpose, Dev fixed in his mind that this Sanyasi was just a person seeking donations. When the same person later asked for a meal, Dev's perception was already clouded by their first interaction. He admitted that he forgot Baba's specific words: "When I come for the meal, two others will come with me." This pre-existing context was the reason he felt such a delusion fell upon him.