What licence?
Microsoft have made source code available before, with a license that said something like: "If you could have seen this source code, and you ever make money out of software in future, Microsoft can sue you for copyright infringement. Something in that source code will bear some remote resemblance to something you are selling, so you must have stolen they idea from Microsoft."
'Open source' is about as definitive as 'real soon now'. It could be anything from GPL to blatant trap.