Seems more like Google is pissed than really desperate for StumbleUpon as a business threat. There must be a behind the door story to this. May be they weren't given a "fair" chance to make an offer?
Not that that explains this case.
Point is the thing google has implemented isn't even close to StumbleUpon. But due to the timing it is getting labeled as that so it makes even more sense that Google might be trying to make a statement here rather than a stumbleupon killer. Of course it could be doing both but it has already done the former.