AFAIK this is not true. While some airlines (LH/LX for example) maintain separate additional inventory for their own M&M members, seats released via Starnet are available to all other *A members for redemption unless the program you are redeeming miles from has chosen to block access to those seats (i.e. prevent agents from seeing the flight).

The new booking platform that agents are now using has blocking capabilities designed into it. Whether or not this is blocking or IT incompetence is hard to know for sure. Agents are no longer allowed to use the 'old' system which allows searching for individual flights. The old system would most likely show availability for the flights you want. They can only request flights by segment and select what is displayed on their flight availability screen.