In a kubernetes scenario, when software says it benefits from lots of disk space for filesystem cache, are they referring to PVCs?
Disk cache refers to memory that is available and used, so that the underlying file system doesn't have to be considered at all. Ephemeral or persistent storage shouldn't matter in regard to that as far as I know.
The page cache makes sure that currently unused memory is used to keep frequently accessed parts of the file system in memory. Since the memory isn't required by any other processes, it's instead used to speed up what's usually the slowest memory - the actual disk. If the whole database / index can be kept in memory instead of having to be read from disk when a query is to be resolved, the performance of most/all databases will greatly increase.