They were able to use CPUs that they would otherwise have to discard, due to defects, by disabling the faulty core.
On most Athlon X3s the user could enable in the BIOS the forth core and usually get a fully functional quad core.
Mine did, also did the pencil trick with the Duron in the past (bridge the 4 L1 links) to unlock it and those overclocked easily so you'd have more bang for your buck.
To be clear, this is how most chips work today. Intel, Nvidia and AMD make entire lineups of CPUs and GPUs with only a handful of different chips, the only difference being features fused off (either because the silicon doesn't handle them well or because they need market segmentation).
I mean, the most recent case was the 1600AF, which was not a new stepping of the Zen 1 based 1600, but an underclocked 2600 (including the instruction set efficiency improvements, cache changes, etc.)
Isn't this how all binning works? If some part of the chip doesn't work, cache, core, fpu, they cut the traces and sell it at a cheaper price point.