No, saying he is a founder is misleading. He played a critical role in Tesla and it surely would have failed if he were not involved but his initial involvement was as an investor in an existing enterprise.
There was a settlement that allows Musk to call himself "founder" in exchange for, I presume, some compensation. There was no legal ruling on facts only a private agreement between parties.
Others had crucial roles as well, but without Musk providing the funding and management, Tesla would have just been another dead electric car company.