So, I asked Cursor to make for me a Python script to turn those .wpost files into Markdown files. And... it did. In the first pass.

Probably because the source code of https://en.wikipedia.org/wiki/Open_Live_Writer at https://github.com/OpenLiveWriter/OpenLiveWriter was in its training data.

I wonder if it supposedly knows the code so well, is it possible to use Cursor to make a port to Avalonia or some other cross-platform GUI toolkit for C#.

Valid. AI is so good that I reach for it even before doing basic searches. It's now easier to have it write a program than search for one, install it, and learn how to use it.