It isn't that room isolation will make things sound bad, per se, but they are for soundproofing, which is different than acoustical treatments.
I will say this. Soundproofing (ok, nothing will truly be "sound proof" unless you bury yourself in a concrete bunker underground and have no door to escape) can help with sound quality though, but not like you think. It is a nice improvement from preventing outside sound from contaminating your room's audio performance. Most people focus on keeping sound IN their room, but dropping the audio level of their "sound floor" by blocking exterior noise helps too. Just keep in mind that without treatments inside the room itself, it won't really matter. That is what, I believe, Andrew is getting at.
Think of it this way. Lets say that you have a great sounding audio system in your car. It obviously sounds better than some cheap-o system in another car. Maybe it is tuned better, has better components, etc, but if you put the windows down (no more "soundproofing" then the audio is completely contaminated with ambient and wind noise and neither will sound great.
Same with audio speakers and gear, and the same with a room. A well treated room can sound bad if you hear the HVAC running next door, or a TV on an adjacent wall.
Any single piece will effect the other. Bad vs good speakers and gear (#1 priority), treated vs. untreated room (should be #2 priority), and isolated vs. non-isolated room (#3 most important, unless you live by train tracks or some other loud environment).
