The whole Haxe ecosystem flies under most people's radar, partly because it evolved from what was once the Flash ecosystem. It never really qualified for the kind of "NEW SHINY THING" announcement that creates hype elsewhere. A lot of people who have written loads of games are peripherally aware of it but have never actually gotten hands-on with it.
I've said before, I about fell out of my chair the first time I looked at the games created with Heaps: Dead Cells and a few other top tier indie games I had been looking at. Still haven't gotten around to learning Haxe though, hopefully soon.
Looking through the documentation, it’s a bit lackluster. So I always got the feeling that the primary users of the engine were the engine developers themselves
That's exactly my thoughts. They had an internal engine they could open source, so they did it. Probably to simplify the management of intellectual property rights between several companies. Making it a popular choice for indie devs has never been the goal.
The feeling is understandable and it could be worrying, but since some years Nicolas seems to be focusing on Shiro games and game dev tooling (heaps, hide). The compiler is developed by the Haxe Foundation and the ecosystem by the community which is not that big but has quite a few talents.