I don't remember that at all. When I had first gotten the Vex Mythoclast, it was during the first time I beat the hard raid on my Hunter in Year 1. This wasn't too far off from when I had finished the raid previous times. At the time, I wasn't even aware of the gun's existence. Again, when I had beaten the Atheon challenge for the first time in the 390 version my pockets were full. I landed in the tower later on surprised to see a 360 or 370 version of the Vex Mythoclast sitting in my mailbox. So no, I can't say that I've ever actually grinded Atheon in the chances that a Mythoclast would pop up.