They could have achieved taxi speed well before the runway end, stowed reversers. F/O starts his after landing flow, selecting flaps up, then arriving at the turn-off point at runway end, find nil braking on a flooded, ungrooved runway... nose wheels just slide when turned. Capt reselects reverse, but too late to prevent over run. I've seen it happen...but on a -200 where much more effective reverse saved the day.
I think BobM2 got the point. If the runway is flooded and you misjudge your speed (at night could happen) you may not stop it even with 15 kts and once out of the concrete there is no more friction.