I suspect it does. When I was evaluating Wasabi years ago, the sales engineers were very interested in knowing what specific kind of data we had and how compressible it was. So my guess (pure speculation) at the time was they use ZFS compression internally but charge customers for the uncompressed size.
If they care about compression at the ZFS level, that means your file is going to be visible to anyone able to log in to the server, because they're relying on (at best) encryption at rest. That's not a great security model for a storage service. You don't want anyone to be able to log in to a server and see your actual files unencrypted.
If they're going to compress/decompress, ideally you'd want them to have that at the point of ingestion, then encrypted, and then store that on the target drive.
That way you can put very strong controls (audit, architecture, software etc) around your encryption and decryption, and be at reduced risk from someone getting access to storage servers.
most of the files that matter, the big ones, are already compressed/high entropy - images, videos, ...