Many Pokemon are both classified the same way mostly because they share, look like or do very similar things. Just like both Ampharos and Lanturn are classified as the light Pokemon due to both sharing the very special trait of having to produce light. As are Goodra, Dragonite and Salamence classified as The Dragon Pokemon.
Groudon being classified the continent Pokemon speaks for itself as its Pokedex entry states:
Said to have expanded the lands by evaporating water with raging heat.
It battled titanically with Kyogre.
This legendary Pokémon is said to represent the land. It went to sleep
after dueling Kyogre.
Basically, Groudon being the reason behind land massa(i.e continents) it is nigh normal that it is classified by what it stands for.
As for Torterra people in ancient times thought that a gigantic Torterra dwelled under the earth
>Ancient people imagined that beneath the ground, a gigantic Torterra dwelled.
Looking at Torterra's design it also basically looks like a walking continent so for this reason it is very likely that it was simply classified by its looks.
So all in all, no one stole any classification, however both Pokemon were classified with the same classification for different reasons that had traits taken from both Pokemon.
However with this, the original classification as "The Continent Pokemon" technically belongs to Groudon.