The story on Jian Yang may have been planted by enemies of Bill English and National but the motivation does not matter.
It does not mean it can be ignored.
What matters most is not whether he taught English to spies in China (he did), whether he once belonged to the Communist Party (some of the best Kiwis have been old Comms) or whether he declared it to the National Party when he first signed up as a list candidate (he apparently did).
it is very interesting, but none of that makes him a spy or a liar.
What matters is whether he told the truth to New Zealand authorities in gaining entry to the country and in gaining citizenship which he gained in 2004.