One personal experience: years ago I worked on a project where I had the task (I will not go into why..) of re-implementing some tricky, non-documented functionality for which I had the source code for another implementation available to study, but also the goal to make it hard to prove from inspection of the code I wrote that it had been inspired by said original code. So in essence, I had the task we postulate the ReactOS developers had, if they indeed were using leaked ntoskrnl source as a reference. Ok, so when I had that task I did things like re-order the declaration of variables; change variable names; think of ways to refactor such that logic was split over a different number of functions; and on and on. Basically: I did a bunch of things designed to NOT permit the kind of "ah ha! : these things are the same" analysis that we see here. You might call this "derivation obfuscation".
So I'm a bit confused: either the ah ha analysis is flawed; or the RactOS devs just didn't care about being found out, right? Because if they had wanted to, it wouldn't have been hard to obscure, at least to some degree, that they were deriving their code from MS's original source.