Officially the UN doesn't allow it. Non officially it doesn't make sense to invest heavily in their airforce when they can't support a full network centric air warfare system.
They can, DPRK is poor (function of it being completely cut off from most of world economy), but it's an industrial state with working education system in full sense.
They're quite aware of what's going on around, and follow up.
Sad truth is that from early-mid 2010s attitude towards Pyongyang was simply dismissive in Beijing(not unlike Moscow in 1990s, though nowhere that bad of course), and given that any Korean state by default is going to be anxious about China (like, 2.5k years of history together) - it added up.
Of course China will come help DPRK if South/US will forget it's place, but Kim's never exactly wanted to become just an obedient buffer state.
That was one of reasons for Russian affair from DPRK side - it's a very political thing.