This is not an issue of statics or dynamics (at least not conventional) but one of vibrations. Tensioning the barrel will indeed reduce the vibration response but so will increasing the wall thickness of the barrel. On the one hand you have a relatively thin-walled barrel that is tensioned and on the other a relatively thick-walled barrel that is not tensioned. Which offers the most favorable vibration response for a particular configuration is not immediately obvious. My guess is that with the two-piece barrels S&W was not after enhanced accuracy but reduced manufacturing cost. Whether they also enhanced accuracy by doing so, is beyond my present day ability to calculate.