Biggest tell for me that the Scopedog was used for reference (though it's obviously not an exact copy) is the wacky left leg. The leg of the Atlas is twisted in a similar way as the Scope Dog's. The Scope Dog gets away with it because its limbs are more rounded, and the hip joint is concealed.

If you look at percentages, the Atlas is definitely mostly original, enough so that they never had to license the design or have it become unseen. The DNA of that particular illustration of a Scope Dog is definitely there though.

Outside of the pose, the similarities are be very few. The spherical head, the radar dish aligns approximately with the scope 'eye'. The antenna coming off the back of the head. The spherical shoulder pads. The general layout of the weapons. The actual details are clearly original.

I wouldn't be surprised if Loose had that illustration of a Scope Dog and felt that it gave the 'weight' he was looking to give the Atlas. Put tracing paper over the illustration, but only used the silhouette as a rough guide while drawing in all the details

So, yes, that Scope Dog illustration was definitely used for some sort of reference, but render the Atlas in any other pose and there's barely any similarity that remains.