Who I have the same questioned this solution

That's a possibility. For the most part, any iPhone as of the iPhone 6 should be universal across all carriers in the USA. The only difference would be is if the phone is an International vs USA Domestic model. The other different would be if the iPhone itself were unlocked. When buying phones through carriers (Verizon being the exception here), the phones are typically locked to the carrier. Buying iPhones through Apple will give you a factory unlocked device, which will accept and work on any SIM/Carrier which the phone has hardware support for. Apple enforces iPhone locking through their Activation Server. This "Activation server" is checked each time the phone is restored or updated.

If a lock was the case, iTunes, if you used that, would have complained about the activation policy of of the device during restoration. I see this all the time when mistaking a Sprint phone for a Verizon phone when inserting a Verizon SIM Card.