Being a founder is about what you do and how much of yourself you are able to put into the company, not when you join. If Chad had hired someone with Paul's skillset and then told him what to do, that is an employer-employee relationship. That doesn't look like what's happening here. They're entering into a partnership as equals to discover and build something new together.

I suppose "partner" is a tempting word to use in this situation, but there may be legal connotations to designating higher-ups as "partners"--in particular, the connotation of being something like an LLP or law firm.