This is a very common misconception about single length irons, and it couldn't be further from the truth. There is no reason a single plane swing should be limited to single length irons, nor a dual plane swing limited to variable length irons. If you have a dual plane swing and you can hit a conventional 7-iron fine with your swing, then there's no reason you wouldn't be able to hit every club in a single length set just as well using the same swing. The only thing that is different from one club to the next is the loft, so there is no logical reason to saying you need a single plane swing to hit them properly when they are essentially all 7-irons with different lofts.