Does anyone else see the irony here? The Dark engineers switched their core language with another because the open source ecosystem wasn't broad enough to cover their needs. And yet they're asking their customers to make the same bet on their proprietary language and platform. I mean just look at this FAQ[1]. There isn't a clear path out of vendor-lock in, a terms of service, or even a guarantee that the platform won't disappear without notice.
OCaml is a fine language, but it's not a wildly popular one. Should the authors have started with, say, Python or JS, they won't have the problem with support from third parties. Please note how their choice was between open-source Rust and proprietary (CORRECTION: also open-source already) F#, both descendants of the ML family.
When you pick a language and start to feel you're overgrowing its ecosystem, you either migrate off of it (as in the post), or start developing it to help it move in the direction you want. In the case of OCaml, Jane Street and Facebook chose the latter route.
UPDATE: Thanks for reminding that F# has been open-sourced: https://github.com/dotnet/fsharp/