IIRC (haven't used Erlang in anger for a decade or so, may be out of date), this is about the only non-Tk option for Erlang UIs, so it isn't _that_ niche. I mean, it's still very niche, but for Erlang users there isn't much choice).
I was surprised to discover that wxWindows still has regular releases. I figured it would be one of those projects that has like a single volunteer doing the occasional bug fix in the evenings and weekends.
It's not just Erlang either. The example uses Elixir which (afaik) is helping increase the ecosystem's popularity.