It's a paper, not a plan. It's not saying you should switch to only wind and solar, it's that you can.

This is a proper plan: https://mitpress.mit.edu/books/electrify

It has a role for existing hydro and nuclear.

Oh sorry, I misread your original comment as saying that plan wouldn't work because it required too much overbuilding, when actually you were saying that it would work! Yes, our existing hydro and nuclear just makes it even easier to switch.