No, I don’t think you’re missing anything Guy. The more gain the basic profile has the less effective the guitar volume pot is for raising the solo level. For totally gained out metal it makes no difference in volume at all but with a Plexi or Tweed Deluxe etc it can provide a nlittle ce volume lift (as Angus has shown).
By the way , the SG clearly is the best guitar because not only Angus used one but also Tony Iommi. Case closed 😁 (I don’t actually own an SG but thats not the point. I only play inferior guitars lke PRS and Musicman 😆)
But Angus doesn't use his volume know for solo's as far as I'm aware...just watched a couple of videos to check and absolutely goes no where near his volume knob. I've seen him use it for quieter passages like Live wire post solo breakdown but not solo's. I suspect he relies on the sound guy riding the faders. Perhaps he did in the early days?
Case back open - Tony played a John Birch and then Jaydee SG's in his early days
BTW I LOVE musicmans!