I've also seen that claim, however as far I've understood things, Kyan and Itosu were more collegues than teacher and student. Kyan's karate looks quite different from Itosu's (this based on looking at karate that is in their direct lineages), since Kyan, due to his small stature, emphasized the shikodachi-zenkutsudachi turn, which is a "trademark" of at least Seibukan. Also, Kyan did not teach the Pinan kata, which, as we all know, were a "trademark" of Itosu's karate.
If I remember correctly, one of my friends told me that the most likely origin of the confusion about Itosu being (one of) Kyan's teachers stems from the fact that they both taught karate at the same school.
I've send a question about this to couple of people, I'll be interested to see what they have to say about this
edit: just got a reply back about this. According to Shimabukuros (Zenryo and his son Zenpo, in this case), Joen Nakazato and Choshin Chibana Kyan was not a student of Itosu