Not true. 100% variable renewable energy (VRE) grids are found to be economically viable almost anywhere on the planet.[1] These are grids which don't require any fossil fuel "firming".
From the IEA report: "Substantiated by in-depth case studies, this report infers that, almost anywhere on the planet, nearly 100% VRE power grids firmly supplying clean power and meeting demand 24/365 are not only possible but would be economically viable, provided that VRE resources are optimally transformed from unconstrained run-of-the weather generation into firm generation."[1]
However, propagandists routinely spread misinformation on firming. For example, they might cite the absolutely absurd LFSCOE which is funded by the energy sector's equivalent of the Center for Indoor Air Research[2][3][4].