With the metadata only on the internal drive, isn't this a SPOF?

Given that it's JBOD over USB I don't think this is aimed at redundancy

Yeah, this was an effort to get around cloud costs for large amounts of 'low value' data that I have but use in my other home servers for processing. I still sync some smaller result sets to an S3 in the cloud for redundancy as well as for CDN uses.

I thought zfs is doing the RAID.

It could be. Author didn't specify. zfs isn't inherently redundant or RAID so it may or may not have redundancy