T-Force: iPhone 18 Promos do not allow for stacking device promotions. Accurate?

I purchased 17 Pro Max last year on EIP, have one year remaining, and since there was a delay in the credit getting setup, actually have a $20 net credit each month which would of course go away if I turned in or paid off the device. My plan was to keep that EIP active and start a new EIP, turning in a different fully paid off iphone for the promo credit. As far as I know, this has always worked, but the T-Force rep said the promo terms for the 18 do not allow this. Can anyone confirm? Perhaps she was misinformed or read the terms incorrectly? The reps exact response to my question about starting a new EIP and leaving my current EIP in place: "you won't be able to stack device promotions on the same line. I took a look at the specific offer document to make sure I wasn't missing anything."