It's only for presentation. He writes in Bear on MacOS and use his script to make backlinks https://github.com/andymatuschak/Bear-Markdown-Export . He made a video presenting how he works, should be somewhere on YT. But if you're asking specifically about that website source code then yes, he was asked about it. He didn't publish it. There are some alternatives: https://github.com/MaggieAppleton/digital-gardeners