Yes, look up the comments on reddit or wherever when it happened everyone feels he seems serious. Its not difficult to tell these things, same way I can tell you are defending him since you are most likely muslim as well and feel obligated to do so eventhough he is very wrong in making death threats to a spokes person because a beer of a sponsor that pays his salary as well is too close to him.
Its not like he is forced to drink it or anything, he is just being a dick and forcing his religious beliefs on others and when he doesnt get what he wants he gets angry and makes death threats.
Hence I feel Jihadi Mustafi is justified
Happy new year by the way