I think the GP was on the right track, but somewhat confused. They obviously don't do this on all models, but some dual-core models are disabled quad-cores. Remember the Athlon X3? That was a binned chip that usually was created from X4s with a broken core. Most buyers didn't mind, and some of them got lucky and were able to re-enable the disabled core.

It seems like the GP might be suggesting that a quad-core CPU will swap in another core when one dies. That doesn't happen. But the binning process allows them to still sell slightly defective silicon with disabled parts (cores, cache), which saves money.

On a related note, a lot of GPUs actually do have a few dozen execution units that are disabled by default and can be swapped in after stress testing at the factory. I believe some can even do that in the wild, but I could be wrong.