It appears to be based on the linked site:
"In particular, ZFS’s advanced architecture addresses two of our key performance concerns: random I/O, and small I/O. In a large cluster environment a Lustre I/O server (OSS) can be expected to generate a random I/O workload. There will be 100’s of threads concurrently accessing different files in the back-end file system. For writes ZFS’s copy-on-write transaction model converts this random workload in to a streaming workload which is critical when using SATA disks. For small I/O, Lustre can leverage a ZIL placed on separate SSD devices to maximize performance."
The LLNL ZFS study has been pretty widely publicized in the HPC community. Lustre uses the filesystem API rather than mounting in. Until now Lustre used ext under-the-hood for data storage, so the performance improvement from ZFS is relative to ext. ext3/4 may very well outperform ZFS on a workstation or small server, but that's not the what Lustre is used for (even their test system is ~900TB).
Disclaimer: I used to work for LLNL.