Did he work for Microsoft though? If yes, I wonder what kind of interaction between him and Mark Russinovich.

No, he did not. Mark Russinovich also started to work on Microsoft only recently.

Mark Mussinovich started working at Microsoft in 2006 according to Wikipedia https://en.wikipedia.org/wiki/Mark_Russinovich

Yep, you are right, https://www.linkedin.com/in/markrussinovich/

But still in 2006 ReactOS already had its 0.3.0 version and 8 years of existence ^) And book Microsoft Windows Internals, Fourth Edition has already been released in 2004

“Recently” is relative. I find 2006 pretty recent.

Now, Anders has been there a while already...

His company got acquired by Microsoft