There’s your answer OP. Call Barclays, ask them to apply promotion for the new iPhones, they will read a disclaimer and you have to verbally agree to their terms to enroll.
Also, Barclays website removed their reward point system for Apple products since Apple Card so Barclays only offer financing option.
I called them and waited forever, but they were able order the watch for me with the 12 month financing. I'm sure I'll pay it off sooner than that, but just wanted the option available.