The widespread consensus among experts who have done DFT is that the preprint doesn't add anything to how they feel about LK-99. https://twitter.com/alexkaplan0/status/1686392015217741825 sums it up: Griffin's paper is neither proof of superconductivity nor even a very strong signal of it. Flat bands can mean many things; in fact, the crystal structure Griffin assumes may have been selected due to their likelihood.