in an economy where the currency is not capital controlled, you can either control the FX, and hope interest rate goes where you want it to go (i.e. Singapore/HK), or you can control the interest rate, and hope FX goes where you want it to go. Andy xie is smart enough to know that if HKMA is working on the FX, the interest rates are just going to move based on market liquidity.
And on the word 'intervene'.. or 'defend'... the actual fact is banks operating in hk will call up hkma and tell them, i have a demand to buy USD at 7.85 hkd, can you sell them to me. HKMA under the Undertaking mechanism just calmly sell the banks the USD against HKD. i wouldn't call it intervene. they don't come into the market and dump $B and $B of USD against HKD just to suppress the currency.