Is tohoto accusative form of ''that''?
No, it is the accusative form of "this":
acc sg -> toho = that
acc sg -> tohoto = this
Why does it become člověka? Is it Acc Ma?
masculine singular accusative: kluk -> kluka
I said "I do not know of this person"? I can see the nuance between that and "I do not know this person" but could my answer also be considered correct?
I am not quite sure it should be accepted.
If you want to surely express that nuance, it would be O tom člověku nevím. Or perhaps O tom člověku jsem neslyšel (I heavent't heard ...) or similar.
I would not be inclined to accept it. The two English sentences have different meanings, and from your comment, it looks like that's true in both languages.