From Wikipedia:

> He came to speak a "kitchen Persian" that allowed him to talk to Persians somewhat. (He was never fluent in Persian as he said he was.) Despite having some love for the Persians, Gobineau was shocked they lacked his racial prejudices and were willing to accept blacks as equals. He criticized Persian society for being too "democratic". Gobineau saw Persia as a land without a future destined to be conquered by the West sooner or later. For him this was a tragedy for the West. He believed Western men would all too easily be seduced by the beautiful Persian women causing more miscegenation to further "corrupt" the West. However, he was obsessed with ancient Persia, seeing in Achaemenid Persia a great and glorious Aryan civilization, now sadly gone. This was to preoccupy him for the rest of his life.

The guy was a terrible person and a documented liar about his knowledge of Persia. Perhaps he had some conversations with locals who genuinely didn't understand why he hated black people, and he thought "ah, these clever Persians, so effortlessly deceiving me when they obviously must be as racist as I am".