The original post does talk about it, but it skips the basics, presumably on the assumption that they're already known.
> will the Linux kernel automatically start using RAM as disk cache if there's enough free space?
Yes, exactly.
> Is it smart enough to prioritize disk cache over infrequently used pages and then know to discard the disk cache if it does need to page stuff back in from swap?
It is, and this is what's controlled by the vm.vfs_cache_pressure sysctl that the post discusses how best to configure.
> will the Linux kernel automatically start using RAM as disk cache if there's enough free space?
Yes, exactly.
> Is it smart enough to prioritize disk cache over infrequently used pages and then know to discard the disk cache if it does need to page stuff back in from swap?
It is, and this is what's controlled by the vm.vfs_cache_pressure sysctl that the post discusses how best to configure.