I'm assuming but the versions are not fully backwards compatible so you can't just ship the latest version, they would need to ship all. There almost ten .NET versions released after the one which ships in Windows. And a new version is released every year.

The author does mention that .NET does have distribution options which don't require the user to install the runtime. You can have it package the full runtime with your build, either as a bunch of files, a self-extracting executable, or a standalone AOT-compiled native executable.

The author mentioned that the AOT-compiled executable is 9 MiB which is unacceptable to them. The other options will need even larger. Personally I don't see 9 MiB as a big deal especially when the author would rather go with Electron which is larger at worst (bundled Chromium) and only inefficient at best (system WebView).