You can have true fullscreen if you set it manually:
1. Open installation folder
2. Open package.json
in a text editor
3. Add "fullscreen": true
to the window section
Save the file and enjoy the game in fullscreen. One minor caveat though: There's no "quit" option in the main menu, so you have to close the game with good old Alt+F4. It will ask you if you want to save though, so you won't lose any progress.
Hope that helps!
Could you please write how the line looks like? I followed your instructions and all I got were errors.