>What's the issue again now, that ECC would have solved?

If the bit flip happened in RAM, the checksum would be of the corrupted data. ECC corrects single bit errors of data on RAM.

>Not to mention that ECC wouldn't help at all with corruption at the disk level anyway.

Yes, using ECC without ZFS, btrfs, ReFS, or checksummed file formats is pretty pointless (unless your application never touches storage).