Among the issues that Cerebras had to overcome was how to connect tiles on a wafer where each tile has to be smaller than a reticle. In normal chips you have a gap on all sides where the diamond saw will cut the wafer into individual dies. Having wires cross that gap requires non standard processing. And the tiles themselves would still be limited to a single reticle (under 35mm on a side), so a multi-reticle M1 would not be easy to design.

That is made in a multi-step process, as far as I understand its design, they made the point of designing it from a completely repetitive structure which is the size of one step.