This seems very much like a prearranged PR event. He says:

"... but to be honest, I’m not sure what I would have made for the deliveries themselves. Postmates recently changed its pricing policy to charge users based on the difficulty of the job, and it declined to disclose how much it pays its couriers for those jobs."

So it is pretty clear that the company is not treating him as an employee, but is giving him special treatment as a journalist.

If he really wanted to show how it feels to be a psotmates employee, he should just get the job as an ordinary joe schnoe, like the lady that wrote "nickel and dimed".