I'm not asking what her parents did (which is indeed explained in the article), I'm asking "why did the author bother to put word 'heiress' in the headline when it doesn't tell the reader any additional information when used by itself".

If the headline read " The daughter at Harvard ..." surely you'd wonder why they used the word "daughter" there, right? "Heiress" without any qualifiers means more or less the same thing, so it's natural to be surprised by the word choice.

"Golden Age film star" descriptor has fairly high information content because there are not that many film stars in the world. Just saying "heiress" without any additional qualifiers (unlike say "heiress to a large fortune" or "heiress to a throne") can literally mean any woman with known parents, so the information content is quite low, which is why it's weird to put it in the headline as if it's supposed to mean anything.