The memory ECC is a tiny bit of logic right next to all other memory logic in the MC, it seems very far fetched to me that there would be a binning process for specifically the instance of that logic being broke but the data logic next door working just fine.

Note that the ECC on the processor's internal data buses and caches is completely independent of whatever ECC the memory controller may or may not use.

> Best I can come up with is binning

Bingo.