Do the nut and bolt need to be removed to adjust the angle of the stem? If not I can't see how that will ever be tight. If yes then I'd say the design still suffers from a flaw which is that tension of the bolt does not engage those teeth.
All that to say that here is how you test to find the area of wobble (because the problem might be a loose headset): straddle your bike with feet on the floor and pinch the intersection of two parts with one hand while you hold your front brake with the other and use your weight to rock the bike forward and backward against the tension of the brake. You'll feel the loose part move. This test is often used to test for loose headsets by squeezing the headset top nut (that big black nut on top of the headset) and the bottom of the stem where it goes through the top nut. I recommend you do this just to make sure it's not the headset. (It looks like I see grease there - was the bike serviced recently?) If you don't feel movement at the headset try pinching the stem and do the rocking.
Whatever the verdict, that stem looks like an engineering short cut and probably will never get tight. I wouldn't be surprised if you sheer that bolt like so many seat tube bolts.