Stoner didn't make the AR15 (scaling down the AR10), Jim Sullivan and Bob Fremont did. Stoner didn't think much of a smaller caliber rifle, he kinda got pushed to design the bullet for it with Remington but that's about it.
It's still his design since the AR15 is a scaled-down AR10 (notwhitstanding the design changes Sullivan did on the AR10 too, i;e: the centered over the bore gas tube/key/cam pin and rear charging handle).
And yes it was an AR15 before it was an XM16 since it was originally an ArmaLite designated rifle.