On zfs there is the ARC (adaptive read cache), on non-zfs systems this "read cache" is called buffer, both reside in memory, so ECC is equally important for both systems.

Rot means changing bits without accessing those bits, and that's ~not possible with zfs, additionally you can enable check-summing IN the ARC (disabled by default), and with that you can say that ECC and "enterprise" quality hardware is even more important for non-ZFS systems.

>Correct data with a faulty checksum will be treated the same as incorrect data with a correct checksum.

There is no such thing as "correct" data, only a block with a correct checksum, if the checksum is not correct, the block is not ok.