Sableye certainly can donk as others have pointed out, but unlike Kingdra, Blissey, Banette, Groudon, etc. it is essentially dead weight if you don't get the T1 knock out because in that instance you will likely not be using it at all. Therefore it pales in comparison to every other donk Pokemon in the format. If the opponent gets any evolution out at all or happens to open with a Basic with 60+ HP (any of the Pixies, Pachirisu, Chatot, Turtwig, Eevee, etc.), Sableye immediately loses its luster. Each of those other donk Pokemon I mentioned retain potency throughout the course of the game (some more so than others) even if the opponent does set up and evolve, meaning that they are worth molding a deck around both in anticipation of a donk and a more drawn out game, whereas Sableye is not.
I think if you're going to run Sableye, the best angle is to try and abuse Impersonate, grabbing Wagers, Felicity's etc. (depending on the deck) and disrupting/setting up from the get-go. Overconfident is simply a nice option to have on standby for the handful of instances where it actually will lead to a T1 KO. It's not the real draw of the card though and anyone who thinks they can rely on Sableye donks to get them to the top cut of any tournament is silly.