As David says, that propaganda line was developed by male college students along with such familiar phrases as "Trust me." Some still believe it! I don't know if any females have tried this particular line.
In reality, the more insulation between you and the cold air, the warmer you'll be. The exception is when there isn't enough room for the warm clothing inside the sleeping bag, and the insulation of both clothing and sleeping bag is compressed and can't fully loft. In that case, you will be colder. The solution is a sleeping bag wide enough to fit over all your puffy insulated outer clothing without compressing either the bag or the puffy clothing.
Standard lightweight backpacking practice is to get a slightly less warm bag than needed and wear your insulating clothing inside, which will take it down another 8-10 degrees F. It never worked for me. I never realized why until the EU's EN13537 ratings came out--the "comfort" level for cold sleepers/women (like me) is about 9*F higher than the "lower limit" rating for warm sleepers/men. It therefore seems logical, after all, that I have to start putting on more clothing at the "comfort level." At the "lower limit" I start getting cold even with all my insulating clothing on. That 8-10*F turns out to be about right after all!
The testing for EN13537 ratings is done with a dummy wearing base layer top and bottoms and a knit camp, and sleeping on a 1 inch foam pad. In other words, the technical folks who developed the standards don't believe that line about sleeping naked, either.