I guess to play devil's advocate from the apparent popular opinion:
Obi-Wan's quote, just like most things in English or any language period, makes the most sense when applied to the immediate context. Anakin expressed a typical, "You're either with me, or against!" line. Obi-Wan's quote is more for pointing out the false dichotomy of Anakin's words - suggesting the galaxy is more flexible than Anakin may think. He suggests the dark side makes people believe in such a rigid way.
And in the end, Obi-Wan could also just be spouting random words. Poor writing is so easy to handwave and retcon.
