Apparently you booked this January stay before registering, i.e. you didn't know the targets you had to hit. This means you didn't stay at an inconvenient hotel to hit the promotion.
I booked after registering and before January - before someone on FT noticed the booking date requirement.
And when I searched back for the confirmation, to see when I had booked it, the only thing I had ever received from IHG was the reminder e-mail, which
looked like a confirmation, with a January date, so I assumed I had already rebooked it.
My fault but oh so frustrating.