I have little experience with IMU, but according to REACH description, Reach has an IMU onboard which will be a down the road enhancement. It's basically a matter of using the best sensor at the time to determine location.
IMU is good for short time interval location and orientaton. RTKLIB appears to reject accelerations based on prnaccelh and prnaccelv configuration. This is platform dependent. You would allow much higher accelerations on small quadcopters than on a luxury yacht. On the other hand, IMU corrects for short term variations.
You can not see it on the Google Map track, but there a few points where the track is determined by SBAS as the best estimate of location.
It is my understanding that the IMU filter software is external to RTKLIB and will be a later addition.