I went through a similar situation a few days ago; I'd booked a reward ticket from my own Aeroplan account, but only have a supplementary CIBC Aerogold infinite card. The check-in agent only asked to see the CIBC card before letting me check my bag for free, even though according to the T&C the benefits are only for the primary cardholder.

Also managed to access the priority check-in line as well as priority boarding by showing the CIBC card. Probably since the benefits are new, but it didn't seem like they were really scrutinizing to see if you're the primary cardholder or whose account the ticket was booked from, if that's possible.