It matters somewhat, if only because some wars ban homemades.
I feel that the designation between modification and homemade on such a level is really only a serious consideration when it comes to the forums. If someone is attempting to use semantics to find a loophole in acceptible weapons policies at wars, I'd rather them just stay at home. Nerf, as a sport, is reliant on the honor system. If we have to create stringent safeguards against it's violation, the fun is probably gone anyway.
As for the boards, I stand by my comment. If the weapon's designation is so unclear as to cause confusion about where it should be posted...than it probably doesn't matter where it is posted.
If you are set on creating a hard line designation, though, the one you mentioned was the one originally discussed for larger wars some years ago. If the air chamber (be it an air tank or plunger tube) is not of the manufactioner's make, the gun can be considered homemade for warring purposes. This, however, is troublesome on a discussion forums where the designations are not designed to limit the power of a weapon but to categorize the work done to it for ease of research and review. A crossbow with a new punger/plunger tube may be more similar to a homemade in power, but likely still retains more nerf parts and contains more modifications than straight up fabrication.