This only applies to Eternamax Eternatus. Base Eternatus was beaten or you could say; mostly relative to Leon's Charizard (This doesn't make Eternatus weak by any means as a wild Pokemon was essentially equal to the strongest trainer-owned Pokemon in the world).Truth.
Eternatus is one of the most powerful Pokémon (especially in the form of Eternamax) that you cannot defeat without the help of Zacian and Zamazenta.
If he is used in the finals (I hope not !!) his defeat by Satoshi will be not only unrealistic, but also the biggest joke of the series.