Why would you assume that when Moog rate it as a unidirectional fiber interface?
Because without very intentionally doing so, all of the optics in a typical fibre or free-space-optics system are symmetric. Why would they put a one-way mirror or circulator in what is intended as a one-way data channel? It's just going to add loss to the system, when if you want a one-way channel, the simple solution is to only put one transmitter on it. It doesn't make a lot of sense for it to be otherwise, so I would assume this is an instance of writing the spec either to an RFP or to what they expect the market expects/understands, not to what the capabilities are. This is, of course, assuming that the bare fibre is what is exposed to you, and there isn't an OEO conversion happening to get through the ring - but if that were the case it seems like there's no reason for using optical at all and doing an electrical interface all the way through would have made more sense. In any case, if it's a completely optical system it is very likely to be bidirectional, but obviously you need to confirm this before buying a dozen of these expensive gadgets.
Edit:
This datasheet is lacking in detail but looks like more or less what you're talking about and says 'bidirectional channel support' and most of their other literature says 'all of the advantages of fibre', and suggests that multiplexing and bidirectional links work, and suggests it can carry any signal (which implies no OEO), so it is almost certain to me that there is nothing here that would prevent 1000base-BX from working over this medium. But yeah, before spending my $50k, I'd be asking them or if I had one I'd buy the $50 SFPs and just try it.