Hello,
First of all, I'm not qualified to give you a definitive answer, and anything I say is only my personal belief. I'm not a lawyer or anything close to one. I'm jsut writing to give you an approximation of an answer while you wait for the proper answer by someone who is qualified.
I believe that the idea of the restriction is to both prevent breaching of "non-compete" clauses in contracts, and to prevent the situation in which the contributions to ReactOS as seen as having been done by "abusing" the knowledge obtained while you have direct access to Microsoft's internal information.
The best ReactOS will be able to say about this, is if your contribution would be legally welcome. You still may want to verify on your end if you would have any troubles by providing such contributions.