I did some research on this, and it turns out you're absolutely correct. I was under the impression that ECC was a requirement for a ZFS cache. It does seem like ECC is highly recommended for ZFS, though, due to the large amount of data it Storrs in memory. I'm not sure I'd feel comfortable using non-ECC memory for ZFS, but it is possible.
Anecdotally, I did have one of my memory modules fail in my TrueNAS server. It detected this, corrected itself, and sent me a warning. I don't know if this would have worked had I been using non-ECC memory.