I defenitly don't get it... If "iu" can be something that is not someone. A lamp, for exemple, "tiu lampo". We don't say "tio lampo", right? Then why "neniu" could not be "nothing" ?
Why do we say "tiu lampo" instead of "tio lampo"?
The correlatives which end in -u are usually assumed to be about people unless there is something else to say otherwise.
Iu does mean "a certain (individual) one (either person or thing), someone (particular)" so tiu lampo is "that particular, individual, lamp." Tiu lampo is obviously not suggesting that the lamp in question is a person with characteristics (radiance, heat, odd body shape) of a lamp, but is still presenting a specific lamp.
Neniu, means "Nobody, no one, not (any) one." It can mean "no specific object," but consider what context we have. Are we expecting a particular object (thing) to appear? if so, then your translation would abide. Since we have no other context to work from, neniu must relate to people. Just as if the sentence was iu aperis we'd be forced, by lack of other context to conclude that it was not some (particular) thing, but a person that appeared.
And we do say tio lampo, just not very often. (and then it's usually wrong, because) The -o correlatives generally refer to nonspecific classes or entities.