Why do you assume that the decision was made by Zlatan to come to United already when Raiola said this? Just because Zlatan ended up at United doesn't mean that he was not looking for other options. In the end, Zlatan thought, United was the best deal and he ended up at United.
Just because a journalist is based out of Italy, anything he says becomes gospel. "Oh but he is one of the most reputed you know". There is no shred of evidence that a deal has been done. BBC not even saying that a deal is being discussed. But of course, they might not have good sources with in the Italian club. They also would not have any good sources within United since Manchester is not in the UK.
Who knows, a deal might be on the cards, but it looks like everyone is being taken for a ride by a tweeting journalist and that tweet is being copied by others ,rehashed / re tweeted and this makes it look like things are happening.