Right you are.
Bob Ludwig, mastering engineer for the ages, mastered 'In Rainbows' and actually commented directly about this topic during a presentation:
(transcription below from
this site)
“This is a very loud record, but it was designed to be that way. This is a record where every distortion, just like Beck Morning Phase, every distortion you hear on there has been carefully thought about. The Beck Morning Phase record, we had done clean mixes of some of those songs. And then when we came out with it, some stereophile people thought it must be from an mp3 because of this and that.
...
Every Radiohead record, we just keep working on it until Nigel’s happy. It’s funny because - I don’t think it’s telling things out of school - some of these records sound nothing like the mix. You’ll be surprised at what some of the mixes sound like, but they’re all malleable. What I ended up with is what he was imagining. So sometimes these records go quickly. Sometimes they take a very long time, it’s very interesting how it goes. But it’s never put out until he’s like, 'That’s it.'"