[I’m not sure if this is down to overzealous rulesharking or oversight]
If there is no official mention of it:
Why am I required to have a basic Pokémon when there isn’t a need for one in my deck (this time)?
Official Rulebook Page 22
Finally, your deck must have at least one Basic Pokémon in it.
Earlier in the document you posted is also “This section assumes readers are familiar with the information presented in the basic Pokémon Trading Card Game Rulebook”