>>50953092This reminds me of something Shudo said about there being plants and plant Pokémon. The wording he uses in the translation "plant based" implies they're not really plants. In fact he goes on to refer to them as animals.
Some of these videos are old I assume. I saw a post dating to 2006 which states videos like these had been part of the Japanese anime for years so they're very old. I'm not sure the dating on this one.
Anyways based off the rough translation, and the use of the word "considered" maybe it means bellsprout is an animal that performs plant like activities such as photosynthesis. It's like Leafeon and grass types like the owl who also perform photosynthesis, it just happens to look a bit more plant like while not being a plant.