This is simply false. First, "base load" powerplants are actually the most troublesome, because they don't adjust for load and predicting base load is actually hard (except when you just use a very small fraction of overall load). Second if you have a large grid (like the US) that is connected (and modern,) you could run the whole grid of variable loads like wind. This would require investment into the grid which nobody seems to want to do. However the technology is there