I think what they want to say is that the editor does not distinguish between typed user input and editor actions like Ctrl-B, Ctrl-R etc. The browser interprets all this as input and if you say “undo”, it does that: it undos all user input, including typed text and Ctrl-keypresses. This, however, is general behaviour of browsers - any browser-based web app behaves this way.
The only solution I see here would be to integrate the undo functionality into the Discourse editor - but this seems like a huge piece of work for a very limited effect - I personally never missed this, neither in Discourse, nor in other forum software…