Sure, but that doesn't negate the fact that, in addition to the filed fares, they can use their direct sales channels to offer web-only specials or some such. Or, for that matter, they can channel a bit of inventory through selected TAs/consolidators. Once I was able to get a very reasonable o/w fare to DXB through STA Travel. Amazingly, the fare was actually in a code for which there existed a r/t filing, and it had the same rules as the r/t fare that used the same booking class (including full miles and upgrade possibilities). That wasn't on BA, and I hadn't asked whether it would be possible with BA (therefore there's no reason for anyone to claim that it's never existed; it may have not existed with BA but it is something that does happen in the industry).
Besides, what's stopping them from filing a couple of really low fares (e.g. one for Y service and one for J service), zeroing out availability for the next 6 months and only opening it 2-3-4 weeks out if the flight looks like it's gonna go empty?
I don't think they are so short of possibilities! They've been playing the revenue management game for the past 30-odd years; it's really strange that they (lots of airlines, not just BA) still get parts of it so horribly wrong!