I'm not confused about the NAT / firewall distinction, but it might be nice if my ISP didn't have a constant, precise idea of exactly how many connected devices I owned. Can that be _inferred_ with IPv4? Yes, but it's fuzzier.
The ISP still doesn't know how many devices are connected, because a lot of those devices are using randomized and rotating IPs for their outbound connections.
Okay but why does this matter? They're your ISP they also have your address, credit card number and a technician has been in your home and also supplied the router in the common case.
The theoretical vague problem here is being used to defend a status quo which has led to complete centralization of Internet traffic because of the difficulty of P2P connectivity due to NAT.