I agree with this. From Microsoft’s point of view, it doesn’t make sense to introduce dirt racing to Motorsport when they’ve already got Horizon. Motorsport can stay focused on road racing, and MS can sell a second game to fans of dirt racing.They both use ForzaTech as the main engine, share models and various other bits like audio and such. But it's doubtful Motorsport will have rally, Horizon has had dirt since Day 1 and Motorsport hasn't followed suit.
Some rumours suggested that RallyCross may make an appearance in the new Forza Motorsport but there's no substance to suggest it's true.