The website does answer your questions:
> It is designed and developed mainly as a hobby OS
A hobby is something that someone does for their personal enjoyment; it isn't any more complicated than that.
That doesn't answer my questions at all; e.g. Linux also started out as a hobby, and yet Linus had a clear position on why he considered his system to be better than e.g. Minix, and why he made certain design decisions. Hobby does not in any way preclude rational, goal-oriented motivation.
And it doesn't imply one, either. Most hobbies are just hobbies, that began as hobbies, and continue to stay hobbies.
Replace programming with any other hobby here -- It would be ridiculous to say, in response to someone showing a hat they they've knit: "Why did you consider it important to knit yet another hat? What is the core benefit of this specific knit hat?"
Not at all. Knitting has a clearly defined purpose: to make a particular garment with a particular shape and pattern so that you can wear it, give it as a gift or sell it (coincidentally, I have detailed first-hand insider knowledge on this topic). But I don't think we need to continue this discussion because it doesn't bring me any closer to the answers to my questions, and apparently you don't have the answers either.
You are asking questions about things that were never asserted in the first place. "Why did the author consider it important to implement yet another unix-like system in C?" Nobody ever said it was!
"There are already so many Unix like operating systems of all sizes written in C... If I were to go to such an effort, I would try another language...It is meanwhile more than clear that it can be done in C...so it would actually be more interesting to explore the suitability of other languages for such a task."
Based on your earlier remarks why not say knitters should use different material or needles to make it more interesting? Why do the same pattern over again when so many knitters have probably knit the very same pattern already.
Because the statements above miss the point of what hobby means to most people.
>> and apparently you can't answer the questions either
No one is required to. Not anyone on HN or the hobbyist in question.