I think it has to do with space. The drydock where the parts are to be assembled have been enlarged from its original size. This took a lot of space from the yard right next to the drydock, which means that there are not enough yard space to fabricate the parts. That's why they have to be fabricated elsewhere, and then moved to the drydock.
Yes the slipway hanger will limit on how much can be done. I was expecting the shipyard to build up to the waterline (or hanger's maximum height) and do as much of the internal work before moving it to the final assembly area. Honestly, I don't understand why this shipyard wanted to follow their British counterpart. They don't have the same constraints as the British.
Link doesn't work.