How is this different from other airports?
Terrorists can get on any plane they want to, with the possible exception in TLV
I departed KUL on a flight to China about two months ago.
Immediately after exit immigration, there is an security queue, where security staff interviews passengers and check their carry-on luggage.
In addition there is another security check directly at the gate.
So any terrorists must have passed those three points without arising any suspicion:
1) Malaysian Exit Immigration
2) Central security check
3) Gate security check
PLUS they must have had an entry permit into China (Chinese Passport, Visa); the 72h/24h transit rule in PEK does usually not apply to citizens from high-risk countries for terrorism.