That sounds correct to me. Very few rates (senior rates often do) allow booking more than 1 room under the discount unless there is more than 1 discount holder present. Now ... this is rarely enforced, but neither is checking eligibility in the first place.

I had a debate once at the Marriott in Berlin on a senior rate (where the rate rules clearly state you can book 2 rooms) and they certainly looked into that and only then accepted it.