There's a lot of optimism in this thread, but does DFT (or any theoretical model really) actually have much predictive value in quantum chemistry? I've always gotten the impression that in this field the proof is in the pudding.
There are so many bad DFT papers out there because it's cheap to do DFT compared to growing and measuring samples carefully. DFT is notoriously unreliable as a predictive tool in strongly correlated systems, though when electron correlations are small it works well. I mean, I want this to be true, but I put little stock in DFT that doesn't calculate observables. So yes, you're right.
The prof who taught us computational chemistry during masters basically said 90% of published results cannot be trusted and most people in this field don't really know what they're doing. Results can look seemingly good and stil be way off from reality, even for very simple molecules. This is a crystal lattice. I take dft and other computational results with a big grain of salt.
GGA-DFT (+ some corrections) used here seems quite ok to me for this system. For more trust into this, I would like similar calculations with other methods to see how similar or different they are. LDA-DFT will most likely not be great (as in most cases), but I would be very interested in some DFT+GW calculations, even though LK99 might not be it's strength.
But it isn't used for its predictive value here, it is used to verify that which is already known (or at least, strongly suggested to be known). That's different than coming up with a compound based on some hunch, this is modeling a compound with a known structure to check that for properties consistent with the expectations.
That's radically different from searching for a compound with particular properties, that is a much more error prone process.
Explaining why is valuable. The band gap described in this paper is common to other high temperature superconductors. While I remain skeptical, this gives a glimmer of hope, and if the material is indeed superconducting, analysis like this is useful in further understanding high temperature superconductors. If it's not superconducting, then this research may yet be interesting -- if the analysis is correct, it would be interesting to know what's different.
