Can you buy duty free alcohol at Hong Kong International Airport for travel into China?
The answer appears to be yes. Unfortunately I can't point you to an official source saying this, but it lines up with various sources (including at least one HK-based one) which detail China's duty free totals without calling out HK-specific limits (other than on some medicines) as well as anecdotal evidence.
Of course. For all intents and purposes, HK is considered a separate part of China. HK<>Mainland flights are international, visa requirements are different, etc...
If you cross the HK-Shenzhen border on foot, or even by ferry, you'll see that there are on both sides of the border duty-free shops. They're a little less developed on the HK side (with possibly the exception of Lowu, where the HK-side duty free shops do brisk business), but all in all Mainland and HK people buy a lot of alcohol and tobacco.
Now, regarding your specific question about HKIA. The only limitation on alcohol purchases in the airport is your wallet. The new DF operator for alcohol and tobacco, which, quite tellingly, happens to be the government-run company CDF (China Duty Free) in joint-venture with a French company, will sell you anything and everything you fancy. You want 12 bottles of Hennessy XO to resell to your friends in Xiamen? No worries. Can I have your credit card and boarding pass please?
The only difficulty will be passing the loot through Chinese Customs at your point of arrival. Not because you're arriving from HK, but because of the quantity. Anything above the DF allowance will be taxed. But stay within the allowance (1.5 liters) and you won't have to pay anything.
According to my own experience (last time was last week), the answer is YES.
The same applies when travelling by boat/ferry, car and even crossing by foot. Alcohol, cosmetics and tobacco are included, depending on your port of exit. And the same applies on the way back (at least for the exit points I have tried: some airports, land and sea).
On a side note, in MOST cases (but not all) both alcohol and tobacco products seem to be much cheaper on the Chinese side than the HK duty-free.