Yes, the binary store has its own temporary directory on the binary store filesystem for temporary storage during creation, but that's not the only relevant case for no-copy optimizations. For the pure use-case of NXP-9361 where you provide an InputStream then yes java.io.tmpdir is not relevant.

The issue with no-copy optimizations is that the initial file containing the binary that is being sent to Nuxeo VCS for storage (through the creation of a Blob for instance) may be created by a third-party library as a file and not a stream, for instance the RichFaces file upload component. For no-copy we want whatever temporary file RichFaces provided us to be used directly, so we have to make sure it's already on the proper filesystem, and we do that by setting java.io.tmpdir.