Reply after filing an online complaint does suggest that I was incorrectly denied entry (bolding mine):

Thank you for your feedback written in our website in relation to your experience of denied entry to our International Golden Lounge due to flying with domestic flight. We apologize for not addressing your concerns in a timely manner and appreciate your patience in allowing us the time to revert to you on your concerns raised.

We regret to learn your inconvenience experiences due to deny from enter our Golden Lounge facilities. We agree that it certainly caused unpleasant journey as you were not able to enjoy the facilities. Our check revealed that you were directed to Domestic Lounge by our staff. That is normal practice for our team to suggest passenger the closest lounge to their departure gate. This is to avoid misconnecting and service breakages. For instance, if passenger arriving from an international sector, connecting onto domestic flight, we would suggest passenger to proceed to domestic lounge due to Custom, Immigration and Quarantine (CIQ) requirement. However, we will never insist on this, and we normally advice passenger to leave the lounge before boarding time as stated on passenger boarding pass for CIQ clearance.

Mr. calvinoeh we truly thank you for taking time to write to us your observations and how we can further improve our services. We thank you for your feedback and we assure you that we will continue in our earnest endeavor to improve and give our customers the service they expected and deserved.

Your patronage support and satisfaction are important to us. Once again we are truly sorry that you did not have the right experience with us.

We hope we have other opportunities to serve you better in your future flights.

Your sincerely
For Malaysia Airlines,