HK/China is not part of ASEAN. You need to add a ticket to KL Malaysia for this to work.

AirAsia is a LCC, the T&C state there might be limited seats per flight available.
Some LCC are bused to the plane from the terminal. Taxes and airport fees are not included. You would also need to pay extra for seat selection, access to premium seats, check baggage allowance and beverages onboard.
The seat pitch is also less.
If you can put up with the limitations and get flights that you want at the time/date that you want it is a great deal.
BE FLEXIBLE!