Once again, I would like remind you that even though Wowpedia can be reliable, don't blindly trust anything without citation and a direct quote in it. Remember that it is written by players - like you or me, and have been wrong / outdated a number of times before. In regards of this matter, please see below explanation.
Thank God that Blizzard cleared this, (hopefully) once and for all, so that people won't be confused with terms that can carry multiple meaning (i.e: defeated, vanquished, etc.) anymore. In Blizzcon 2018 - 8 months after Chronicle 3 was published, they clarified:
Originally Posted by Blizzcon 2018 Interview
Q: Third Chronicles have information about Old Gods. Every Old God, except N’Zoth, have been beaten, but not “killed”. It is true? Are Old Gods like C’Thun or Yogg-Saron still alive?
AA: I would say that we should consider them dead. But! As with all things in World of Warcraft and in general in the Warcraft universe, death is not always final. Therefore, if there is the coming of the Old Gods, or some precursor of the Old Gods appear from the eternal darkness, from the Void, for example, if there is any way or opportunity to return them, I imagine that this may well happen".
There you go, they are dead. However, like everything else in WoW, being dead doesn't guarantee that they will be gone forever - and neither will that stop them from interacting with the world in some manners. This is NOT limited to just the Old Gods - we have seen Medivh (before rezzed) and MU Gul'dan (in form of merely a skull) interacting with the world before.
To be honest, knowing the Old-Gods-can't-die group, I guess I shouldn't be surprised that after so many indications, both in and outside the game, people are still believing in the crap that we only killed their physical manifestations so they aren't dead and such
They are just as dead as Medivh before Aegwynn resurrected him, as Kael'thas, Arthas, Onyxia, Nelfarion, and many other bosses we killed (ironically, though, probably less dead than the "immortal" demons who got killed inside the Nether). Most of the time (with some exceptions), we only killed the, well, "physical manifestation" of bosses - that didn't prevent them from being dead.