Hacker News Reader

Not really, if you read carefully they are saying that their OpenGL renderer is very similar to the DirectX 9 one (probably because feature parity between these two APIs), but they have difficulty matching DirectX 11 features with OpenGL.

Like a database ORM which can have Postgres/MySQL as the backend, but then you want to replace Postgres with BigTable. It will be very hard to keep the MySQL support.

They probably have their own abstarction layer that implements the subset of DX9 they are using on top of opengl. This is very common on linux ports as it is hard to justify the cost of a full rewrite of the graphic engine.

Feral Interactive has managed to port many games to Vulkan. They even updated their port of Mad Max to Vulkan after release.

Yes, and I own copies of many of their ports :)

It's called wine. But I have no idea if they were using it.

Wine implements DX11. So that is not the issue why the company is dropping support for Linux and Mac.