Also found https://www.osnews.com/story/15175/review-reactos/ with some comments from ReactOS developer.
Finally, after some deep searching I found https://web.archive.org/web/20070106124749if_/http://www.reactos.org:80/wiki/index.php/TinyKRNL it is no longer available on their website. From one side they say that they had relationship with Tinykrnl and the latter used dirty reverse engineering methods. On the other side they said they couldn't directly utilize it. I also found that Tinykrnl and ReactOS had at least two common developers.
I don't know what to think about it. I'm puzzled.