Benzene exists as a compound with 2 resonance forms. So it's not a mixture because Benzene does not "pick" one resonance. Resonance forms are there for you to be able to draw them, but in reality any compound is in all their resonance forms.
Yeah, I'd agree with that completely. The fact that benzene is neither one resonance form or the other is true. It's a hybrid of the two. My speculation is the original statement simply serves to explain benzene within structural formula parameters. From that perspective, benzene can be understood as being in equilibrium between the two structures.