Shorter barrel= Less accuracy
In my humble experience, in general....yes, but that is when you are talking about SBR's that are AEGs.
A barrel that is 430mm will stabilize the round reasonably enough to keep you on target (depending on velocity/bb weight/atmospheric conditions). The longer barrel "can" improve the round stability and trajectory, but I wouldn't trade ease of movement/drag for a 700mm barrel.
I built a KJ m700 and cut the barrel down to a realistic length. It was HPA/CO2 and was easily as accurate as the full length counterpart. (it ended up at 510mm...or 480mm...I don't remember)
Regardless, there is merit in the correction that KA-BAR is grilling you on, but he just sucks at being polite about it. In some respects you are correct, but in others specifically this whole single shot forum most blanket statements concerning parts and accuracy are up to serious debate.