I felt like submitting a “bug report” on github was a little heavy-handed for this, since technically if you follow the instructions it correctly leads you to download the IDE, but…
On the web IDE, when you select “Show Code for Arduino”, the footer of the modal windows says
Just connect your device via USB and click on “Upload” button
When you click on the Upload button, it then instructs you to download the desktop IDE. It would be clearer to say in that footer something like…
Download the desktop IDE for more advanced functionality