I don't think I've ever heard this. The alumni of the university has always, from my experience, been used to refer to everyone that graduated; gender playing no role at all.

Saying “always” based on your experience with a word that is over 2000 years old without knowing the history is a great way to be wrong. In this case, if there is a group of multiple people, the correct word is alumni, unless the entire group is female, when the word becomes alumnae. Alumni is correct even for a hypothetical group with a billion women and 1 man. If there is just 1 person, it is alumnus for a male and alumna for a female. Most universities would use alumni because there is always 1 male in the group and they want to use the plural. A women’s only university would use alumnae. That said, English speakers have a tendency to mispronounce alumni as alumnae, so trying to maintain a minimum understanding of how to use the word correctly might be a losing battle.

It’s one of those “well actually” things that the Latin nerds would point out. So the Latin nerds who went into college administration decided to change it to be a clearly English derivation.