I would say that, in-universe, Marla could have been completely incorrect about Khan probably being a Sikh. We as the audience are supposed to take this line of dialog at face value and accept that Khan had a Sikh background.
But in the episode, nobody else confirms McGivers' conjecture. As far as we know, it could just be a wild guess on her part, as Khan doesn't have certain identifiable traditional accoutrements such as turban, beard, and metal bracelet. So how did she get "probably a Sikh" from looking at him in the sleeping chamber for five seconds? Further, while all male Sikh initiates have the surname Singh, there are also non-Sikhs who also have the name.
Of course, it was basically a throwaway line to give the character an "exotic" air for the typical American casual viewing audience of the day, who was not sitting there with a pile of reference materials (or the internet at their fingertips) ready to fact-check and nitpick stuff like this like we are doing fifty years later.
Kor