The article is just about some guy's opinion on starters. Also, nobody reads the whole sentences here.
"What do you want to bet that he'll be able to learn Pin Missile?"
That's like saying:
"What do you want to bet that
Flareon will not get Flare Blitz?"
It doesn't confirm anything. It's just some guy saying that he thinks Chespin will learn Pin Missile because of flavor reasons. It doesn't mean Chespin will be a Grass/Bug type. I repeat, this article is just some guy's opinion. There are tons of opinions like these all around here, Serebii, etc.