This is not Satire or a joke he actually have been saying this for weeks and months, and even yesterday he reiterated it in a number of interviews for British and international media. The clip below if from June 4th, from a press conference he gave AFTER the latest London terrorist attacks.
Go to 07:08
He even repeated the same slogan today when he gave an interview to ITV stating once again that "London is the safest global city in the world".
http://www.itv.com/goodmorningbritain/news/sadiq-khan-on-islamic-radicals-policing-cuts-and-trump-feudTo call London, a city where terror attacks have become almost a common feature of the city, the "safest global city in the world", makes him either one of the stupidest or the most deluded people not only in the UK but in the whole world.