That claim comes courtesy of Ben Cousins, manager of ngmoco Sweden and former general manager of EA's Easy Studio. "It is general knowledge in the Stockholm dev scene that Mirror's Edge 2 is in production at DICE," he tweeted earlier today, though he's provided no further details. It's worth noting that Cousins has previously been cautioned by EA after the publisher was concerned that he might disclose "trade secrets and confidential information." Kotaku has a copy of the correspondence, which dates back to July 2011.
Cousins was responding to the Eurogamer story that DICE did not want to "become a Battlefield factory," though Eurogamer themselves were quoting from an earlier piece from Official Xbox Magazine in which EA Games vice-president Patrick Soderlund said "the DICE guys are roughly 300 people in the Stockholm studio; not all of them are working on Battlefield things."
EA have previously hinted that the time might be right for a second Mirror's Edge, but have never confirmed that a sequel is in any stage of development.