This tutorial is not the best, on a couple of notes:
There is not, and never has been, a "CommonProxy". Per definition, a Minecraft-proxy can only be either Client-side only, or Server-Side, the very antonym of "common". "CommonProxy" is a misnomer and based on faulty logic.
Never call ForgeRegistries#<REGISTRY>#register. Use the RegistryEvent's for this. ForgeRegistries is, and I quote the documentation, "...a central place to access registries directly... queries and iterations can use this"
Likely due to #2.
The code for setting the models, is never actually called. Just because you have the method doesn't mean it's gonna get used. Use the ModelRegistryEvent, and for each of your items with IHasModel, call IHasModel::registerModel.