I don't think you necessarily have to read and/or write fic to be in a fandom, but I am sure that every fan has their own definition of what constitutes being in a fandom. I considered myself to be in a fandom years ago when I didn't write fic and barely read it. I did love discussing the "source material" with people online.

I always liked discussing books and films, but growing up I was banned from entertainment magazines and almost all merchandise. To be fair the entertainment industry in pretty much every country that has one (or maybe every country) has some toxic elements.