Also it's not obvious that it works the other way. The only requirement of the flash reset operation is that it consistently causes bits to read as 1 and that

Also it's not obvious that it works the other way. The only requirement of the flash reset operation is that it consistently causes bits to read as 1 and that the normal write operation allows them to read as 0 again; it doesn't actually follow from that that an erased-but-not-reprogrammed block will have cell charge levels that are indistinguishable from each other based on the pre-erasure charge level.