No, it's not a terrible argument. Anyone can have a pitch or idea. That does not mean it's automatically a viable product/service or a viable business.

It's a valid question, how do we define "founder"? To play devil's advocate, I'm curious if the people who think Musk didn't co-found Tesla also agree Aaron Schwartz didn't co-found Reddit. He joined later, after reddit was already incorporated by Hoffman and Ohanian.

In business, "founder" is already an honorary title. It has no inherent power. Co-founders often ensure they get C-suite positions as a company grows, have stock/shares, or other legal powers, but none of those are guaranteed just by being a "founder". So practically, there's no difference between calling Musk a "co-founder" versus "honorary co-founder." Let's just focus on calling him a piece of shit for the very definitive and obvious things we can point to.