Leaving that RAM for ZFS L2 ARC perhaps? I don't think they would use Illumos as the hypervisor OS without also using OpenZFS with it. They also need some for management, the control UI, a DB for metrics and more.

Btw. if I count correctly, they have 20 SSD slots per node (if a node is full width) and 16 nodes. They would need 2 TB to reach 1 PB of "raw" capacity with the obvious redundancy overhead of ~ 20%.

It is also quite possible, they don't use ZFS at all and use e.g. Ceph or something like it but I don't think that is the case, because that wouldn't be cantrillian. :-) E.g. using Minio, they can provide something S3 like on top of a cluster of ZFS storage nodes too but they most likely get better latency with local ZFS and not a distributed filesystem. Financial institutions especially seem to be part of the target here and there latency can be king.