I think it’s a ripoff of the Windows Research Kernel that Microsoft licensed to universities under an agreement that was obviously violated by some, as the code has been uploaded to numerous places, some of it on GitHub.
I glanced at the ReactOS code tree, and in my opinion, there is absolutely no way on earth this was written from a clean sheet only from the available public documentation.
For starters, there is no such thing as public documentation for the NT kernel internals. The only printed documentation consists of two black binders where every page are labeled Microsoft Confidential.
Many internal data structures and internal functions, not exported anywhere and not part of the public symbols, have the exact same names as they appear in the Research Kernel (which, by the way, is quite obsolete). There is an almost surely zero probability that this happened, at that scale, by accident.
A more sinister scenario (given the amount of code beyond what can be readily found in a few minutes of Googling) would be that ReactOS originated from one of the several leaks that happened in the past.
Now, I′m not a lawyer. Why this is allowed to exist I don’t know. Probably because it ends up being something like a baby mosquito on the back of an giant elephant.
If any of the presumed authors wants to chime in and explain the similitudes, I’m happy to change my mind, but be ready to answer some though questions about the origins your coding and naming styles, and all the design choices that you made and why you ended up architecting and writing things the way you say you did ;-)