He's unbelievable! Its obviously not meant to be a serious cadenza, so I don't think its a problem at all. Bear in mind that opera singers of mozart's day would sometimes insert whole arias from different operas in order to show off what they could do - i don't think mozart would object too much to someone having such fun with his music, especially as he probably played around just as much but in a different way in his own performances just for a laugh (what a git!)
Great musicianship, great to see the orchestra inspired by it too.