> There's existing golang versions of jsonata, so this could have been achieved with those libraries too in theory

The only one I found (jsonata-go) is a port of JSONata 1.x, while the gnata library they've published is compatible with the 2.x syntax. Guess that's why.

Looking at the releases, it looks like JSONata's 2.1.0 release from July 2025 added the `?:` and `??` syntax, and there hasn't been an update to the syntax since January 2020's 1.8.0 release that added `%`