It's not just mental health that the ketone diet could contribute to, but other aspects of immunity as well. A recent paper showed that a normally-dormant subset of T-cells was activated during ketosis, and that this led to protection of mice from the influenza virus:

Ketogenic diet activates protective γδ T cell responses against influenza virus infection.

https://immunology.sciencemag.org/content/4/41/eaav2026