To a reasonable approximation, cats only fall in zero-g.
It's relative.
I think this post is using a technically-wrong-but-still-more-or-less-gets-the-point-across definition of "zero-g"
Considering cats fall feet first and buttered toast butter side down you can power your spaceship by taping buttered toast to your cat's back and attaching the cat toast contraption to the rotor of a generator
That experiment has been performed and indeed produces surplus energy, according to reliable sources :-)