Eh, I have heard SAs say this but I don't buy it. The bench should be one of the easier items to make. Its a metal frame, wire mesh interior, foam filling, plastic edge, leather covering. It's the same materials used in the 5/7 seat benches.
If anything the 6-seat should be harder to manufacture as it as a motorized track system where the 5-seat just screws in.
My educated guess - they are pushing for a high Q1 throughput and want to discourage buyers from the cheaper models. They can produce more M3/MY where the MX slows down factory throughput.