I'm highly suspect. For Spanish, I've always heard piñas rather than ananás...
Now, regarding usage, you're completely right. Nobody uses ananás; everybody says piña.
(Funny: When describing the ananás plant, the DRAE says about its fruit: "fruto grande en forma de piña".)
The entry for Ananas at https://es.wikipedia.org/wiki/Ananas says it's used for the genus which includes pineapple.
The equivalent English word for the genus is Ananas - https://en.wikipedia.org/wiki/Ananas .
Read the answers and you'll see the claim is based on cherry-picking.