The problem with that theory is that you lay on half the sleeping bag, thereby squashing all the fibres. To compensate for this we use a sleeping mat to add extra insulation, which everyone agrees is a top idea. When we suggest adding extra insulation to the top of the system, by way of extra clothes, we have posters suggesting the whole thing effectively becomes a fridge! It doesn't of course.
If you are wearing appropriate wicking type underwear, you would never generate enough sweat to cause a moisture problem, unless you were doing something very "vigorous" inside the bag. Just laying there asleep would not cause a problem.
If anyone doesn't believe us try this:
Experiment
1x wool thermal type glove
1x pair of Ski mittens
2x freezer bags
2x thermometers
1x bucket of ice water.
On one hand put on glove, slip thermometer into glove and put on mittens, put other thermometer into remaining mitten then put on freezer bags. Dunk hands (encased in mittens and bags) into Ice water. At 30 second intervals record temperature next to hand. Stop when you are bored.
One hand represents sleeping bag, no clothes
the other represents sleeping bag with clothes.
let us know what you find.