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.
Hmmmm interesting
She put me on hold for a very long time and when she came back she was quite adamant.
The part that doesn’t make sense to me is that the Marriott activity doesn’t include explanations— there are separate entries for regular earning points, hotels etc without any explicit breakdown.