Yes, the story has been confirmed by many reliable sources and the developers have taken responsibility.
This story from the BBC explains how:
It's like starting a fire with a parabolic mirror.
The location of the focal point, F, depends on the curvature of the parabola. A shallow curve, like that of the building, places the focal point further away from the mirror.
The Guardian mentions that same architect's work has done this before, in Las Vegas:
it has happened before – in a project by the very same architect... they realised this special glass was in fact burning their guests as they lounged by the swimming pool beneath the soaring mirrored facades
CNN confirms the burning happened:
A skyscraper whose reflected rays have the power to melt cars...
And, the developers have taken responsibility:
Canary Wharf Construction and Land Securities have been very good and agreed to pay for the damage, and accept that there is an issue which they will resolve.