I'm willing to accept I'm missing something, but overbuilding is not a sufficient approach because you still need to have generation available at night when there is no wind. Doesn't matter how much you overbuild solar and wind you can't overcome the problem of no sun and no wind.

That study isn't hiding anything, it is an attempt to estimate how much storage is required. If you adjust the solar/wind capacity (i.e. overbuild), you'll reduce the storage requirements but there are diminishing returns resulting in very expensive systems long before your solved the storage problem.

If we had grid-scale storage that was economical, it should be very easy to build a production system to demonstrate that capability. I've not seen any examples. And it certainly seems wise to actually build a system that demonstrates the viability of grid-scale storage before decommissioning base load generating capacity.