Based on my personal experience, my opinion is that the limitation is due quite only to the fact that the down-going aileron ( that of the up-going wing) will cause an added load on an oalready g-loaded wing. In other words, if you make a 6-g pull-up, together with a co-ordinated roll to right (one where no side-slip is induced), the left wing ( already loaded with 6-g from the pull-uo) receives an added equivalent load of 3-g's due to the rapidly down-going left aileron, creating an equivalent load of 9-g's.