"Self made" defies precise definition. If you start with $999,999 and earn $1 by your own efforts, a strictly precise definition for "self made" would imply you were a "self made" millionaire. However it would also apply to somebody who started with $1 and earned $999,999 by his own efforts. Hence using "self made" in both cases renders the phrase so broad as to be empty of meaning.
It doesn't need a precise definition. I think most people will understand what you mean when calling someone a self-made millionaire. That's all that matters: the phrase conveys a meaning.
I'm sure it has a meaning, just probably not one you'd agree with?
I've had this debate with others before, and "self-made" overall tends to mean that an individual did something in isolation. I.e. It plays into the narrative that "no man is an island" and that we all owe our gratitude to the society we were part of, as if the society had some direct doing in it.
