It is so obvious the crew was already at taxi speed well before they even got to the end of the runway. The FO calmly gave a braking report mid field. Doubt he'd do that careening down the runway while unable to stop. Don't understand why anyone would even bring up touchdown zone, wind or RVR with that in mind.
When they went to turn off the runway at the end, the plane probably didn't turn. Engine idle thrust was likely enough to overcome braking action, and they went off the end at slow taxi speed. Happens to one airline or another every year. This is the risk we ALL take flying into those conditions. The braking action report was wrong. That's a big deal. We need that to be right or at least close. Don't give a crew "good" when its F'ing "NIL." Come on!
Only way to avoid this is to never fly in snowing conditions or anytime ice is even possible. Cancel my flights with pay protection, you wont hear any arguments from me. Otherwise, if you want year round flying, this is going to happen, and we don't need second guessing from management or worse, fellow aviators, who should know better. Might be you next time Iceman. Think about it.