I don't know why you're tying yourself in knots trying to attribute that to compression, when to me the Occam's Razor explanation is that a guitarist that spent the vast majority of his adult life in front of high-decibel amplification (and who most likely has the attendant hearing loss to match) created an Atmos mix that mirrors what he hears in the original stereo mix.
All the original issues of the various mixes of this album (stereo, UK/US quad) have a rolled-off top end, which I think is at least partially a function of how the album was recorded, basically in the corridors and rooms of a hotel, hardly ideal conditions for optimal sound quality. Although it can be exacerbated by things like too much reverb or echo, muddiness or murkiness is almost entirely a symptom of EQ - if guitars or keyboards or vocals sound muddy they probably need more presence (5kHz range) or sparkle/air (10-12kHz range). If the guy doing the mixing can't hear well in those ranges to start with he's much less likely to perceive anything as being 'wrong'. The average upper range hearing of someone at age 40 is 15kHz, and by age 50 it declines to 12kHz - what do you think Dweezil's is at age 54 with the extra mileage he's put on his ears standing in front of guitar amps for 30 years?
If you want to hear what mixing stage over-compression sounds like, listen to the guitars in the rear speakers on the Foo Fighters' Low from the One by One 5.1 mix. Granted we're using qualitative and not quantitative words here (and 'writing about music is like dancing about architecture' as Dweezil's dad once said) but to me the result of that is that the guitars have a razor-sharp hyper-realistic texture where you feel like you can hear everything, sort of like the musical equivalent of adding MSG to food.
That isn't to say that Dweezil didn't use compression in the mixing stage - it's been an essential part of rock and popular music mixing since the 1950's - but it's not the universal evil that only makes things sound worse, despite what some mastering engineers and their acolytes might tell you. Is it possible that there's some compression somewhere in the chain that hasn't helped the situation? Sure, but it's not a nuclear football that would drastically alter the tonality of a recording and turn a good mix into a bad one, or vice versa.