Everything I said is true for at least for Linux, where files are memory-mapped into the process space, and swapped page counts relate to anonymous pages (look at /proc/vmstat for the gory details; note that pgpgin/out and pswpin/out increment under different conditions).
OS X may admittedly be different. It's unfortunate that it doesn't expose more counters to help show what's really going on.
As for "reported pageouts are dirty memory pages being tossed" - where are they being tossed to, if not the filesystem?
