That's an interesting point. I wonder whether Fuji have found a way to do TTL flash metering with the non-DX units?
This doesn't seem to be the case for the Kodak DCS600 series cameras which are based on the F5. According to the SB-28DX manual the TTL auto flash modes aren't supported - Auto Aperture mode which is controlled by the sensor on the flash is used instead.
It may be Fuji are simply doing the same thing - relying on the A mode so it doesn't matter if you use a non-DX flash.