Appeal to authority is a weak argument. We are trying to use reason to figure out what's going on, not close the discussion by saying that we are not the entity that gets to decide such things.
Everyone knows no country besides the US has ever experienced a recession, since the NBER didn't declare it. And even for the US, they've only happened since the establishment of the NBER, since they weren't around to declare recessions before they existed and thus there were by definition no recessions. We should just abolish the NBER and thus ensure there will never be another recession.
NBER has declared that the definition of a recession is when NBER has declared a recession. NBER has declared that disagreements with that definition are clearly invalid, since NBER is the source of the official NBER definition of recession.
The "two quarters of negative growth" was only ever a rule of thumb that mostly fit, never the official definition.