I think they basically made a v1, didn't find product market fit due to some realized issues, and then scaled down the company to just one person still working on it as a result since they have significant engineering work to do before another real attempt can be made at growth.
I can't find a good writeup of this anywhere, but you can see the v2 roadmap here which outlines where they are going: https://roadmap.darklang.com/goals-of-dark-v2.html as well as posts here about the F# rewrite: https://blog.darklang.com/author/paul/