I need a help.

I’m looking for the meaning of the word ‘ex’ in the American Heritage Dictionary 4th edition
ex(1) (ěks)
prep.
Not including; without: a stock price ex dividend.
From, but not having graduated with, the class of: a Columbia alumnus, ex '70.

According to the sentence cited from the dictionary, an ex '70 is a 'not having graduated with the class of ‘70’
So why can an 'not having graduated with the class of ‘70’ be a alumnus?

‘alumnus’ is someone who has left a school, college or university after completing their studies there.

Thanks.

He graduated late, as I did. I was class of '64, but didn’t graduate until '65. I took an extra year of courses before I had enough credits in any one field to graduate in it.