The NEC paints with a pretty broad brush in this area. While I can't think up too many scenarios where a fence would have to be bonded it could happen. The NEC does not have a requirement that the conductive material has to be electrically connected to the PV system, only that it be normally non-current carrying, electrically conductive, and likely to become energized. Since there can be many non-current carrying conductive metal objects in the area it is the last requirement that pretty much drives the determination. Is it likely to become energized? Is it possible to energize a fence, yes. Is the PV system likely to energize it, that would be hard to do. About the only thing I could think of would be if there were overhead electrical conductors in the PV system that went over the fence. In that case if the conductors fell on the fence then they could energize it. Would this be a likely thing to happen, maybe. I have had an energized 12kV line dropped 5ft from me and I would not have thought that likely before it happened.
So I would not automatically assume that the fence is out of the running for bonding, just extremely unlikely.