According to Camerapedia: Fujica GW690, the GW690 III was released in February 1992. There is a photo.net post from Nov 2001 that describes a FujiFilm representative stating that the camera had been discontinued.
If the camera was discontinued prior to 2000, all GW690 III cameras, regardless of serial number, would have been manufactured in the "mid-nineties". There would still be a greater than 80% chance that any given camera was made in the "mid-nineties" if production had continued into 2000-2001.
As you've noted, knowing the range of values can be helpful, assuming they are assigned sequentially, starting with 1. The blog author probably thought that the serial number didn't look too high and guessed that it was probably made in the 1990s since he states:
It is not easily clear in any searches I have performed when Fuji decided to discontinue these cameras, but I would hazard a guess at the early 2000s.