So, I was watching one of the Sinnoh shows, and the Piplup knows Whirlpool, and it is not aviable in Pokemon diamond, pearl, and platinum.
The anime does not conform to the rules of the games. Pokémon frequently learn moves naturally that would normally be TM, HM or Tutor moves in the games. Examples are Pikachu's Iron Tail and Buneary's Ice Beam.
Source: I know things. Powerful things.
The games and anime have different logic, so even if a move isn't available in the games, it could be available in the anime.
Hope this helps!
you want to teach it whirlpool
everything is possible
use a cheat to help it learn