The documentation claims that in Linux to use Code Mirror's multiple selection you have to use alt. While alt is the multiple selection modifier, to create multiple cursors you have to use ctrl either alone or with ctrl+alt

The current explanation is incorrect and incomplete for Linux. Also have to account that multiple selection works on a per line basis. And a markdown paragraph can be both a really long line or multiple lines. Given that neither Markdown nor Zettlr enforces neither definition, this can create the impression that multi cursor doesn't work.