What an awful story, I'm sorry you had to endure that. My undergrad is in applied math and I would never even dream of calling myself a "mathematician", so, yeah, tells you a lot about the guy.
I hear a lot of people with a BS in Comp Science that call themselves computer scientists which seems mostly inaccurate too.
It's just odd, these titles and what qualifies you to call yourself a thing.
Mathematician, OTOH, does kinda usually imply that, or at least implies you do something that likely requires beyond undergrad level education.