I wonder if they figure that out every time the CPU boots, or at the factory. At this scale, maybe it makes sense to do it all in parallel at boot. Or, even dynamically during runtime.
There may be edge case cores that sort of work, and then won't work at different temps, or after aging?
EDIT: to add a bit more and possibly address the original question (which I think keveman may have misunderstood), there will usually be some hardware dedicated to controlling the chip's redundancy. Part of that is often a OTP fuse-type thing that can be programmed during wafer test to indicate parts of the chip that don't work. Something (software or hardware) will read that during boot and not use those parts of the chip.
With this many cores it seems like the probability that a core dies during a multi-hour job (or in case it's used for inference, during a very long-lived realtime job) is pretty high, so the software in all layers would need to handle this kind of exception. They probably don't, today, since we haven't seen a 400k core chip before.