What was claimed
The IMF says that the UK will be the fastest growing economy in the G7 in 2016, whereas it had predicted that a Brexit vote would be immediately bad for the economy.
It’s true that the IMF puts the UK at the top of the G7 for growth in 2016. It had envisaged scenarios where the UK economy suffered soon after a vote to leave the EU. That hasn’t happened, although the IMF still thinks there’s trouble coming.