I think your travel agency means that if they sell enough cabins on your Majesty departure (usually 8), then they can form a group where the get benefits from Royal Caribbean, like an onboard credit.

You should push your travel agency to include the onboard credit regardless. The onboard credit should be advertised on their website and if it is and the agency does not offer it to, its false advertising. Be careful to check the terms & conditions though. Its often restricted to certain cabin types (like balcony, suites, etc).