Rain like this is rare buts it’s shut down exit like this before, this isn’t unprecedented. Even during a normal year they shut down entry/exit when sandstorms reduce visibility - often for a couple of hours every few days.
But why rain? There's a road leading up there, right? (or at least somewhere near the festival grounds). Most people will come by car or RV. They can still drive on that road I'd think?
If the festival grounds turn to mud, well... maybe a little detour to get back on that road.
(personally I'd be in no hurry to leave. But if you wanted to, what's holding you back? Organisers saying "road closed"?)