How did pikachu's thunder defeat geodude in EP 292?

Okay so ash was battling against the gym leader roxanne and ash told pikachu to use thunder on geodude.The thunder was effective and geodude was unable to battle.Electric type moves have no effect on ground but in this episode it has been effective.Was it an error?

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beacuse it’s in the anime.in the anime all things can happen.Example: we’ve seen this taillow surviving pikachu’s thunder.

Many times in the anime, many characters explain how and why electric moves have no effect on ground Pokémon and how and why they are weak against grass.

One of the best times they explained this was in this episode:https://www.dailymotion.com/video/x553h9e

Season: Advanced generation seriesEpisode 007, Tree’s a Crowdhttps://bulbapedia.bulbagarden.net/wiki/AG007

Go for 21:40 minutes

Brock will explain!!

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Max ask why Pikachu thunderbolt work normally against Treecko, since Treecko was a grass type and electric moves was weak against grass types. Brock then will explain that the electric move worked normally because it hit Treecko when he was jumping!!Brock explain that electric moves are weak against grass and have no effect on ground Pokémon because the eletricity flow throught the body of the Pokémon and hit the ground or tree right away, dispersing the energy, them the grass Pokémon take minor damage and the ground Pokémon take no damage!But when the Pokémon is not on the ground, like hoving or jumping, like in the case of Treecko, the move work normally because the Pokémon is not on the ground, so the energy has not to defuse on it!–

In the fight against Roxane, Ash waited the right time to hit Geodude with a electric move, when Geodude was hoving.