They expect that some of the cores on the wafer will fail, so they have redundant links all throughout the chip, so they can seal off/turn off any cores that fail and still have enough cores to do useful work.
The fabric can effectively route signals diagonally to work around an individual defective core, with a displacement of one position for cores in the same row from that defect over to the nearest spare core. That's how they get away with a claimed "1–1.5%" of spare cores.
