Let's back up: When they started buying B1s, they couldn't believe that each seat took three days to make. They marketed the hell out of the seats and collected lots of up-front front money from potential customers. When they were told the seats couldn't be made any faster, they begged to be sent the molds so they could be made on site in SoCal. Doing so would have given them the keys to castle, so to speak, and each request was rebuffed.
They then made a mold of a B1, called it the 'T2' and proffered all sorts of nonsensical claims with respect to manufacturing processes and weight. Looking at photographs of their seats refute their weight claims and 'expert' manufacturing processes.
Look at the holes for the shoulder belt cutouts:
Notice how thick the composite is. That is an area where the seating surface and seat back meet. With two halves making a whole that thick, there is no way those seats weigh as little as advertised. Additionally, they do not have the manufacturing capability to produce parts that come from the mold with a glossy surface as shown on their seats. They are coated with a lacquer that adds weight. The problem comes with the fact that the finish is prone to scratching whereas an epoxy or polyester resins are not as susceptible to that.
Additionally, the weights they claim are for bare seats. The funny part about that is that very few people buy bare seats. The upholstery can add two to three kilos to the weight. When a customer is delivered a seat heavier than advertised, it's blamed on the upholstery. The figure you quote for the B5 is a fully-upholstered seat.
Until you see those seats weighed by someone who has no dog in the fight, don't believe the hype.