Well, in such a scenario I'm also making two assumptions:
That the modders are indeed being paid by Bethesda to work full-time on it since it's meant to be sold when finished (not unlike Skyrim Special Edition, only made by modders)
There's a way to add code and assets from existing mods to their remake. If they have to make it all from scratch then yeah, a year is not enough.
I wonder what would happen if Bethesda gave a few of the best modders full access to the game's code and to the latest version of the engine. I bet they could come up with a current gen remake in under a year without the involvement of a single Bethesda dev.