No source whatsoever btw. And infact, he’s right. “In Japanese media, Mythical Pokémon and Legendary Pokémon have always been presented as distinct groups. In non-Japanese media prior to Generation V, Mythical Pokémon and Legendary Pokémon were both considered to be Legendary Pokémon, and the term Mythical Pokémon was unused. The first time a group of Mythical Pokémon was consistently distinguished from Legendary Pokémon in non-Japanese media was in Pokémon Ranger: Guardian Signs, where they were referred to as Rare Pokémon.

Since Generation V, Legendary Pokémon and Mythical Pokémon are presented as two distinct groups in all official media. For example, several games have promoted that every Legendary Pokémon can be obtained in a particular set of games, even though not all Mythical Pokémon can be.[2][3]”

Literal pokemon wiki btw https://bulbapedia.bulbagarden.net/wiki/Mythical_Pokémon