They could bring features like RSR or FMF to Polaris and Vega.

It does a different thing, FSR upscales while this duplicates the framerate by adding an interpolated frame in-between two real frames. It can also be used in conjunction with FSR.

If you mean because FSR3 has motion interpolation too, the difference is that AMD Fluid Motion Frames works on a drivers level, it doesn’t has to be implemented into each game and can even be used on videos, similarly to FSR and RSR, or CAS and RIS.