What makes you say that "As for the Q/A, not breaking the game and "functioning as a whole", they most likely skimp on theses steps"? I don't see the evidence of that, if we look at Sage for example- it wasn't missing any critical skills on delivery, not was it far above nor way below the other healers. Did it get some adjustments late? yes- but they were relatively minor.

I would also agree that job designers likely don't start off from absolutely zero, however there is nonetheless a design/discovery process, reviews, presentations, discussions, approvals- before anyone gets into the actual detailed design, coding, QA, etc so no , I honestly doubt that 40 day estimate follows Square's methodology.