Did Moctezuma really believe Cortes was a returning god? In my Curriculum there's two different books on the subject and they give contradicting accounts.
There's always room for discussion, but perhaps these previous topics will answer your inquiry.
It is possible that the individual citizen believed it, but although the arrival of Cortes would have been a massive cultural shock to the Incans Aztecs, Moteczuma probably did not believe that Cortes was a God.
This source here credits the tale to Franciscan Bernardino de Sahagu, who wrote it 50 years after the actual invasion. It is worth noting that Bernardino was one of Cortes' men who came with him.
It was important propaganda for the Spanish empire to portray Cortes as a holy or powerful warrior of sorts, in order to show Lutheran christians in Europe that Spain was a major Catholic power now.
The more likely explanation is that Franciscan Bernardino de Sahagu exaggerated or straight up fabricated this tale for European Christians.
The conquistadors' were surprisingly literate, and had a very high literacy rate for their time, compared to most Spaniards. However, depictions of Cortes vary greatly, from some depicting him as a God (such as the source I mentioned), to others condemning him (even within Spain) as an immoral and debased mercenary. Therefore take most sources on Cortes with a pinch of salt.
I find it incredibly unlikely that Moctezuma honestly believed that Cortes was a God, but that said, Cortes most definitely exploited his situation as a foreign Spaniard to his advantage.