Just in case the author shows up here, there's a confusing little typo in "Passing argv to our program":

    _start:
     popq %rdi
     movq %rsp, %rdi
     callq main

the second %rdi should be %rsi, which is confirmed by the explanatory test:

> If it's really that easy all we will need to do is add movq %rsp, %rsi after the popq %rdi we just added.

EDIT: now fixed