You are correct. All the rooms are rectangular with non-permanent treatment. The concept behind The Atrium Studio was to have vibe and comfort take precedence over perfect acoustics. Keeping in mind that the studio is in Tommy's main residence, we didn't want to have the studio look or feel out of place from the rest of the house, so we decided to keep the shell pretty basic when it came to shape and simply treat it after the fact with acoustic materials as needed. The actual construction is far from basic residential. All the rooms are a "room-within-a-room" construction with floating floors in the live rooms and custom built acoustic doors. The sound-proofing is top-notch throughout the studio and although not theoretically perfect, the results we're getting in the studio speak for themselves!
We are in the process of adding some custom-designed diffusion in the drum-room to tame it slightly (it's very reflective as you can imagine), but still keep it sounding big and lively!
Tommy's had the opportunity to work in some of the best purpose built studios in the world and if you gave him the choice between a rectangular room that felt great, looked cool and was comfortable vs. an angular, weird looking, cramped room that had perfect acoustics... I'd bet he'd pick the former 10 out of 10 times. Keep in mind some of the best sounding (and selling) albums of all time have been recorded in less than acoustically ideal environments (houses, hotels, castles, basements, guest houses, etc.).