Though it was something AMD was working on last time I checked FreeSync doesn't support non-exclusive fullscreen modes and G-SYNC's implementation isn't entirely trouble free thought it does mostly work (it can still sometimes get a bit confused if you have several applications running that make use of GPU acceleration/compositing features).
Just an observation, why else would the game have an option for freesync unless it was needed to be activated within the game in the case of low abstraction api's?
would need someone to test it though. But i believe with DX11 games it currently needs exclusive full screen to run as it has to detect the game to sync it. but if the game engine itself is directly activating freesync then i guess it would be different. same as with mGPU working in window mode when using low abstraction API's since the game engine directly controls it.