postgres cache vs ZFS cache

I'm looking at some performance tuning for a server running FreeBSD and the
DB on top of ZFS.

I keep reading the recommendation to turn off the primary data cache in ZFS
since postgres does its own caching. The recommendations also say to set
effective_cache_size to a significant amount of the RAM. The claim is that
this avoids double caching of the data.

My understanding from my past research is that effective_cache_size only
tells postgres how big the OS cache of the data on disk is, so there really
is no double cache. It is just a hint so it knows roughly how much to
expect the OS to hold on to.

Based on this understanding, I believe this recommendation to turn off the
ZFS primary cache for postgres data to be wrong. Or is there some other
place postgres is caching the data beyond the shared memory pool?

I find it mentioned here:
http://open-zfs.org/wiki/Performance_tuning#PostgreSQL among other places.

Thanks for any clarifications and advice here.