I think the guy is right. Metroid Prime 3 has a very fast and precise aiming solution because of the motion controls. Even tough the Wiimote had some lag, Metroid Prime 3 is in my opinion one of the games that implemented them the best. It was pretty smooth and accurate.“It was straightforward to update MP1 and 2 to Motion controls, but converting MP3 to normal controls would be a herculean effort, as it is scripted very specifically using volumetric triggers to detect the motion in precise manners to do specific switches, and the bosses are tuned to take into account the ease of gestural aiming.”
You can always replace this by the lock-on feature of the previous games, but there are still instances where it might not fully do the job. And you have to stay still to aim manually in Metroid Prime 1 and 2. So it is clear that some parts of the games would be problematic and would need to be redone differently.
He is not saying that it is impossible. However a lot of work is required and I can understand this.