If you tested that in court you would almost certainly find that the receptionist has not been granted the authority to enter the office into legally-binding contracts.
I don't think that's so much the point, though. It's a fun "hack". And while the receptionist may not have that authority, the end result was that he also did not sign their document, so he achieved his initial goal of not signing a document he did not agree to.
They have two choices: Treat him as if he signed nothing binding at all, or accept his version of the document.
