PAO's usually do very well in resisting rust, unless perhaps the car is stored for exceptionally long periods so the gravity pull leavs exposed metal on the cam , crank, etc.
I would like to view the sample in our Lab's high power microscopes to measure the size od the iron particles, as well as their shape - as someone else pointed out, if the particles are small, they really do little damage, and may have sidestepped the filter - either by a partially fractured filter, or a stuck bypass valve, or are just too small for the filter, esp if it is something like a K&N, whose filter media provides high flow at a somewhat reduced micron capture capability.
The shape might tell something about where they came from, and how they were spawned - "sheared" asperities, or true wear from a film breakdown (which you would see first on the cam lobes).