Actual the solar thermal plants that exist DO have fossil fuel backup .....your understanding of solar thermal comes from reading a few articles not actual experience
The development of thermal storage is a relatively new addition to solar thermal electric technology, but it has been installed and tested well past the "pilot plant" scale, and full-scale plants incorporating it are being constructed.
And no, I am not involved directly in the technology, but I've studied WAY more than "reading a few articles" over the last thirty years. But then, I doubt that you have any better experience base.