Given how unrealistic reviving Wunderlist is (almost everyone of the original employees is working on other things inside Microsoft or has left), this ought to be a PR stunt, right? He might as well start from scratch at this point.
Relatively speaking, building a small team of capable engineers is much easier than getting a business, especially in a competitive space like TODO lists, to 13+ million users (their numbers in 2015 at the time of Microsoft acquisition).
Today they likely would start from scratch anyway, the old app is now decade old or so. The real worth is all in the brand. Bringing back a somewhat beloved app, innovate it a bit here and there, this will bring them some attention and money, maybe enough to sell it again some years in the future.