When I looked two years ago, SSB wasn't documented as a protocol or storage format much, and instead focused squarely on usage from JS code and a million of mic

When I looked two years ago, SSB wasn't documented as a protocol or storage format much, and instead focused squarely on usage from JS code and a million of micro-dependencies via npm, making it hard to tell data formats from implementation details. I haven't checked since, but unless the situation has substantially improved since, it's no surprise that the JS implementation is the only one out there.