People keep referring to Kaguya's third eye as a Rinnegan when the manga has not once called her eye the Rinnegan.
people are just making assumptions that it is a Rinnegan because Sasuke said a Rinnegan genjutsu can likely be broken with another Rinnegan.
but people do not realize that Madara had the Rinnegan so any genjutsu cast with his eyes are instantly a Rinnegan genjutsu. Hagoromo never said she had a Rinnegan he said "beside the byakugan my mother had the power of the Sharingan" he did not say she had the power of the sharingan and Rinnegan he simply said Sharingan. i think its annoying when people keep saying she has the Rinnegan
until they specifically come out and say she had the Rinnegan they should simply refer to her third eye as her third eye or her Sharingan. some people try to justify them calling it a Rinnegan by saying that hagoromo said she had the power of the sharingan and not that she had the sharingan. thats absurd logic. saying someone has the power of the sharingan is the same thing as having the sharingan, you can not have the power of the sharingan with out having the sharingan. all Rinnegan are sharingan but not all sharingan are Rinnegan.
so in the end they should just call her third eye what the manga has called it(a sharingan)