This article is only saying that ZFS can mitigate disk data corruption caused by bad RAM, mainly through using checksums, not that it can completely prevent disk data corruption.

Also, it does not talk about the scenario where the in-RAM data being corrupted does not come with checksum. For example, data received from the network by the NFS/SMB server to be written to a file, before it gets passed to ZFS. This data is stored somewhere in RAM by the NFS/SMB server without any checksum before it gets passed on to ZFS. ZFS does not do any work here to detect or repair the corruption.

So, ZFS does not prevent on-disk data corruption caused by bad RAM, and only mitigates it. Using ECC RAM results in a huge relative reduction of such corruption, even though some people may consider the non-ECC probability to be already low enough.