The preflight RAIM check is just a prediction. It's possible that they did a "live" check at the start of the approach, and found that RAIM was available. The avionics will also let you know real time if RAIM is unavailable.
Not saying it was right- just raising a few possibilities.
What was the wx at the time? If it was VMC, they may have been able to fly the procedure without RAIM.