Again router requirements came out before NAT, you're not going to find anything in that RFC about something else they hadn't written yet. You have to look at the RFCs for NAT to see they are referred to as routers e.g. starting with
https://tools.ietf.org/html/rfc1631:
"2. Overview of NAT
The design presented in this memo is called NAT, for Network Address Translator. NAT is a router function that can be configured as shown in figure 1. Only the stub border router requires modifications."
The IETF collectively aren't big fans of NAT as a good solution but that hasn't stopped multiple standards track NAT RFCs per year. v6 only increased this with all of the transitional mode NAT types (46, 64, 464, 646).