I hoped it was clear in the blog post: this is a very specific case where I only needed to convert coordinates to h3 index, with this specific goal this transpiled version outperform the cgo version.
The comment "probably due to Go runtime scaling on multiple cores" was me still seeing very close performance with GOMAXPROCS set to 1, but observing the process behaving differently than the pure C version.
The fact it was not thread safe has been discussed already on Reddit and is not relevant nor an issue when used in a batch scenario like this.
ccgo is awesome, but feel free to discuss something else ;)