I have no opinion on the accuracy of the agent's explanation, but it is plausible. For a long while some business owners have complained that American Express's system make it easy for employees to see the owner's rewards balance, and from that to estimate aggregate spending. An effective remedy would not only remove this information from paper/PDF statements, but also from employee logins.

However, I really would expect to see this fixed first on cards that earn Membership Rewards, and then copied over to the co-branded cards.