Wouldn't you have to test the physical file and not the url ?
Unless I am mistaken, that rule tests the url
super-cache is rewriting a regular url to the .html.gz
But the point is moot anyway, I tested by accessing a direct .html.gz and it doesn't work either
though I have to find that new setting in 4.x where .htaccess is not processed immediately - I vaguely remember that being added, so maybe it's not in effect