They are using it. Review Board is a successful project that's been around for a long time, and it's solving a problem they had. One of the most common workflows with Review Board for source code reviews is to use the RBTools command line tools to post or update reviews. The cli would be the one generating the diff (although it supports uploading diffs that you generate iirc.) I haven't looked into the details, but I assume RBTools can generate DiffX diffs which is probably easier for the backend to process. (E - from what chipx86 has said in some of his posts here, they have been using it for several years now)

I don't really see this as pushing anything, more as documentation of something they did for themselves, but are also willing to provide to anyone else if they want to use it. Same as how the source code for the core Review Board product is available for anyone.

If you're happy with the diff format you're using in your workflow, keep using that. No one's twisting your arm to switch to DiffX.