First I do not believe Dr. Neal and Dr. Mann are measuring the same thing. If you look closely at the swing analysis report
that Ham referenced you will notice that 'Setup Foundations' (address) has a perspective that is pretty clearly vertically above the golfer. Dr. Mann has that same perspective for address and impact but the referenced report has a very different perspective for downswing/impact. You'll also notice that the report does not have the target line indicator (the black line) for impact that it has for setup. Add to this that the technology of the device is that it will measure rotation along its axis and there is a lot of evidence that shoulder rotation at impact for Dr. Neal is on the tilted axis of the upper torso and not relative to a horizontal plane (which as you noted will vary with torso lean).
If this is the case then directing people to have their shoulders 25-50 deg open to the target
(horizontal plane) is totally incorrect.
I think when a golfer hears 'shoulder rotation' and 'open' they think of the horizontal plane and the target. I think a golfer hearing 'trunk rotation' is more likely to ask what is meant as well as to think of it on the tilted plane of the torso. The term 'open' in my opinion is still problematic; better to say 'past level' I think.