Since we'll be staying there just after New Year, I've immediately contacted the hotel. This is the response:
Greetings from the Courtyard by Marriott Hong Kong Sha Tin!
Our Executive Lounge opens from 06:00 - 00:00 daily , we didn't serve breakfast there for now as our Momo Cafe is just renewed & we would like our guests can have better / more food choices & have better service at our Momo Cafe. Our lounge will still have the happy hour time from 17:30 - 21:30 daily.
Translation: providing breakfast in the lounge is more expensive and we'll try to see if we can get away with "just" providing breakfast in the restaurant.
I'm writing "just" because obviously breakfast in the restaurant is much superior to what can be provided in the lounge. However, the lounge will be more quiet and much faster if you just want to grab something and be ready to leave for the day.
This time I hope they'll continue to have this arrangement until we're staying there since we'll be staying for leisure with enough time to enjoy an extensive breakfast.
HTB.