This is a grey zone for sure, and I have in the past made a booking (only the one) for my uncle using the discount (before I was made aware of the "card holder must travel" bit) though he had no trouble with his trip.
Since discovering this, I havent exploited the loophole further. However, I am not the primary card holder, rather my husband - the other Mr. Amargrossa - is.
Though I have a card in my own name (linked to his account/card) and since that does technically make me a card holder, I've had no guilt about offsetting the additional YQ on my UUAs to CW with the WTP tickets bought using cardofferu