I don't really get that. Corporations don't generally do things because they "get pissed", they do things because because it's economically advantageous in some way or another, no?
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.
Not that that explains this case.