No, they do not act as diffusers.
Uhhh, they actually do, though maybe not ideally.....and they can also act as absorbers, though again, maybe not ideally.
Doing a thorough acoustic inventory room is tricky business that often serves up surprising results. The square footage can add up quickly and can be your friend or your worst enemy. Its all about the frequencies attenuated or accentuated by those square feet and in what measure eh.