Usually the teeth are attached somehow to the fabric strip. I think the strip has a ridge at the edge where the teeth go, and the teeth are clamped over that ridge to hold them in place. Then the fabric strip is easy to sew onto a garment. It looks like the new design has only the ridge, hence it's called a string, and is hard to sew onto a garment.

"Without them, YKK had to rethink every step of production

The teeth were redesigned, the manufacturing process rewritten, and new machinery developed to attach the closure to garments. “The absence of the tape posed various production challenges,” Nishizaki says."