That is not true.
In Michigan, a long gun under 30" in length with the stock collapsed or folded is a considered a pistol by some absurd logic. Federal law is 26".
I have a Kel-Tec Sub 2000 that is a long gun by federal law, but I had to register it as a pistol because it is 29" or so in length (just slightly less than 30").
A 16" barreled AR even with a collapsable CAR or M4 stock, is 30 inches in length. Therefore, it is a rifle by federal AND Michigan law. Not a SBR, that deals with barrel length on a shoulder-stocked weapon, not on overall length.
If you were to use an example of an AK type rifle, and put a side or underfolding stock, then it becomes less than 30" length overall with the stock folded. So you would have to register it as a pistol.
AR15s are fine with standard collapsable stocks. I do not own a stubby stock or some of the other collapsable stocks (LMT, VLTOR, etc), but I suspect they are not shorter than a standard CAR or M4 collapsable stock.