GSM Locked iPhone? Or Not?

I have iPhone 8 Plus (Model A1897) which is supposed to be a GSM only phone. I unlocked this phone from my carrier, T-Mobile, after paying it off. I inserted an activated Verizon SIM into the phone the other day and I received service. Does Apple's designation of A1897 and A1905 mean they are only incompatible with CDMA networks until being unlocked by the carrier? If someone could please clarify that would be great.