Since the IP address of my PC is on a subnet local to my phone and my PC, different from the subnet used between my phone and the carrier, I would say NAT is used.
I'm telling you, and you can contact the network technicians at your carrier to verify this, that there is, indeed, NAT involved (in fact, we agree on this part). Carrier grade NAT on the carrier's network, not on your phone.
The IP space of the AdHoc LAN created when you wi-fi tether is, of course, local and quite likely on a different subnet than the one over which your phone talks to your carrier. That's how 1:1 routing works. 1 IP internal to your LAN routes to 1 IP external to your LAN. That is not NAT. It's also not pandering to the carriers; it's the very same thing dumb phones that allow tethering do. Back before 2G, it used to be circuit-switched data, your phone actually acted as a modem and you'd dial in, but tether-capable phones have been doing 1:1 routing for at least 2 decades now and today's phones use the same infrastructure. If your phone is doing NAT, it is doing you a disservice, because your carrier is, as well. The reasons for this have already been explained, but I'll take another go since you apparently completely missed that.
By the time phones technically gained the capability to perform NAT locally, we were already running out of IP4 addresses. There is not enough address space remaining to give each prone two separate public IP4 addresses, one for mobile and one for tethered data, the infrastructure is already in place to not need to do local NAT anyway, and doing local NAT would strain the CPU, RAM, and battery of your mobile device. As a result, mobile OSes don't implement NAT as we commonly talk about it, because it would be a battery life and performance killer and other solutions already exist; that's not to pander to the carriers, that's to make their products look better on paper and perform better in reality.
Actually... and I'm going to leave the above post as-is because it's all pertinent information... technically, yes, 1:1 routing involves translating network addresses. It's not NAT as we commonly talk about it, where one public IP is shared among many private devices, but yes, it is translation of network addresses, so from a very pedantic standpoint, it's NAT and yes, it is done on your phone. It lacks all of the connection and endpoint tracking functionality we commonly refer to as NAT, though; it's literally "anything coming in to this IP forwards to that one" and is commonly done in the radio chip itself, which certainly does not support what we commonly refer to as NAT.