Great question! Full disclosure, answer copied from a another comment:

It's similar to JuiceFS, but JuiceFS writes and reads data from S3 in a proprietary block format. This means that you cannot connect JuiceFS to existing data sets in S3, and you cannot use data written through JuiceFS from the S3 API directly. On the other hand, Regatta reads and writes data to S3 using it's native format -- so you can do these things!