All of the 3D printed parts slot into each other by quite a way so they are held together by more than just the weld. The head tube weld is a strange one and one I don't fully get Tom's explanation for. Apparently there is a limit on how long he can 3D print a head tube which means for tall riders he has to produce the head tube in 2 separate 3d parts and then join them. He made the decision that to keep the look consistent across all his bike they would all now have split head tubes. It seems an odd decision to aesthetically compromise the majority of his bikes to ensure that they looked consistent with the few big frames he makes.
On most of his bikes these days people are getting 3D printed bars, stems and fork crowns which means he can completely hide all the cabling and really fine tune everything for each individual rider. As he's not tied to any other manufacturers parts he really can go full custom.