Complete BS. It seemed clear what the offer was, exactly as you have described.I called them a few days ago because I hadn't received the 5% bonus either. They explained that the 5% was only given to people who linked their Flybuys and Velocity accounts during the promotion period for the first time (ever). I explained that the fine print seemed to say that the 15% bonus was for people who linked their accounts for the first time ever, but the 5% bonus was available to anyone.
They escalated it to a supervisor but stood their ground and told me that I wasn't eligible. In the end I gave up.