There's no "disagreement" over the definition of the word "founder". It only has one definition. And it doesn't apply to Elon Musk. That's just a fact. There is no "gray area" here, that leaves room for debate
When someone knowingly and intentionally misrepresents the facts...that's called a "lie". Elon Musk is lying about being a "co-founder" of Tesla. It doesn't matter who else is willing to repeat that lie...it's never going to be "true".