Gauss made that computation after he already had Newton’s laws and Kepler’s work behind him. He knew that the result had to be very close to an ellipse and that any deviation was going to be from nearby planets, and he knew the rough order of magnitude from that. If he had just had the small amount of data he had, and had no idea what the orbit should look like he wouldn’t have been able to do so.
Gauss made that computation after he already had Newton’s laws and Kepler’s work behind him. He knew that the result had to be very close to an ellipse and that any deviation was going to be from nearby planets, and he knew the rough order of magnitude from that. If he had just had the small amount of data he had, and had no idea what the orbit should look like he wouldn’t have been able to do so.