What else did we hear on the shows about the Talosians? Or the Organians? Or the Metron? Does that mean that they don't exist either? Other than a mention of the Organian Treaty in Trouble with Tribbles you could make a case that Errand of Mercy never happened since Klingons and the Federation seemed to be able to attack each other with ease (Day of the Dove, More Tribbles, More Troubles, all of TNG/DS9/VOY). Never saw any sign of the Organians stepping in to stop things. I'd say the Slavers and the Kzinti are just as valid in the Trek universe as the Organians are.