I am not 100% sure, but, in fact, if they were NOT on diplomatic status but just on a routine visa, you may very likely qualify to apply for citizenship.
I know of a friend whose parents were here in the early 70s as PhD students. My friend was born here during that time and she has Canadian citizenship from being born here. She and her parents left at some point in the seventies and moved overseas for a job, but my friend still has Canadian citizenship (and also that of her parents' country since in her case that country allows for dual citizenship). Incidentally, she has never really lived or studied here after she left back then - she studied in the US and UK and works overseas.