That guide is a little outdated. The button is now called “Add Medium”. There is no tool to split a medium, but here’s a workaround:
1. Click “Add Medium”
2. Go to the tab “Existing Medium”
3. Search for the same release and select it
4. Click “Add Medium”
5. Remove tracks 12 – 16 from medium 1 and 1 – 11 from medium 2
6. Because of this, the recordings are still correct so you can go to the “Edit Note” tab, enter an edit note and submit your edits.
EDIT: By the way, that medium search thing seems to use the indexed search, which is only updated every three hours. When I tried to do this just now I could only find the digital release that was already on MusicBrainz. So if you try this now, don’t forget to change the medium type to “CD”.