Sorry for the slow reply, the 4th of July is my favorite holiday and I take my screwing-around-and-not-doing-anything pretty seriously.
That tool holder is meant to be used on a lathe and although it may work for some applications the carbide tool itself is not intended to used while rotating. There is no relief on the carbide tool. The front edge will cut a very nice thread profile as it first enters the material but as it rotates the back edge comes in contact as well. In the buffer tube of an AR I doubt that it would be a big issue since the aluminum will give a bit.
Anyway, what he said - most of it anyway. The tool will cut the profile and the inward rotation, but then on the continuing rotation out of the cut it will cut the thread straight instead of being relieved at the pitch angle of the thread - true threadmills have this relief ground into them to not cut where they shouldn't (they don't make perfect threads either). I don't agree with the concept of the aluminum giving a bit in relation to the cutter, if that is what was meant.
The way for an average joe to check the thread shape is to cut a test piece and then section it so you can check the thread across the side. But if the threads you've made work for you and you're using them on low stress joints then go with it I guess.