I think that my definition of fanworks follows that old saw about pornography, where I know it when I see it. Not hearing your song, going strictly by your description, I would be comfortable if I saw it listed as a fanwork. Filks seem inherently transformative of the song to me.

And it being about coffee doesn't seem like an uncrossable hurdle to me, even if it's a fandom of one - I've seen enough fanworks to inanimate objects to be ok with it, anthropomorphic or not. And I'm not comfortable deciding a minimum number of people/works to define a fandom, media based or not.

(And I, personally, am a big fan of coffee!)

ETA: Not that I would necessarily define coffee as a media fandom if pressed, but the filk part covers that for me.