They'll aim to catch the logic failures and memory failures during wafer test at the factory. This testing is done at room temperature and at hot. There are margins built in to allow for ageing. If they want to ship a decent product they'll also need to repeat the memory testing every boot, and ideally during runtime but maybe the latter isn't a big deal for something like this.

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.