A clever use of history.replaceState and some special characters (to make it less obvious that the state's been altered) - "view source" in Chrome operates on the currently active URL, which after the replaceState has executed is the "not that easily" page.
You can also reproduce the behavior by navigating back, then forwards again - you'll see the "not that easily" page that way, as well (as the "try and view the source" page has been replaced in your history list).
Just to add to this, the "special characters" are "%E2%80%AE" which is the escaped form of the UTF-8 "Right-To-Left Override" control character (which effectively reverses the display of text).
Even without the RTL directive, they might have used something like sourcе.html — which contains a non-ASCII glyph. So while RTL is problematic, I'm not prepared to call it a failure in that generally problematic space of unifying human communication.
You can also reproduce the behavior by navigating back, then forwards again - you'll see the "not that easily" page that way, as well (as the "try and view the source" page has been replaced in your history list).