The CPU Tier List isn’t maintained by Amiibo Doctor, it’s actually its own separate research! The SmashCPU scene covers every Smash game and many, many other games as well. You can join their Discord server here!
The CPU Tier List is produced by aggregating the votes of the individuals who run “CPU Tournaments”, in which level 9 CPU opponents fight each other in bracket, just like a normal tournament.
February 2022 CPU Tier List
January 2021 CPU Tier List
The above tier list is the January 2021 CPU tier list for Smash Ultimate. This is the early 2020 incarnation: