Mine is definitely a promotion.

"Register now for your exclusive offer and earn Diamond Elite status until the end of 2025 when you stay 10 total nights between 1 October and 31 December 2024."

My last night to be within 10 nights of qualifying normally was on 15th September, crediting on 17th September. Whilst the promotion would clearly have been live internally at that point, you'd have thought IHG would run the e-mails through whatever filters they've set up a bit later than that.