The entry in Duden is wrong.
The declination of »die Totale« is exactly the same as of »die Horizontale« or »die Vertikale«. The declination class to which these words belong is called »adjektivische Deklination«, and all words that are declined by this scheme are nominalized adjectives.
This declination class is complicated, because it depends on the kind of determiner you use together with this noun.
You can find this scheme here, in Wiktionary, where it is shown for the word » Vertikale« (unfortunately, Wiktionary has no entry for »Totale«). You can compare it with the entry in Duden, and will see, that Duden shows for this word the same wrong declination that it shows for »Totale«.