During the Alabasta arc when the crew was crossing the desert, Luffy, Zoro and Chopper fell down a hole which led to a big dome-like structure. In the dome, there was a huge stone with Poneglyphs written on it. None of them knew what it was much less its meaning. It was dismissed. What I'm wondering is if those Poneglyphs was important and whether or not it had some clues about the Void Century?
Since the Poneglyph in the desert was a non-canon episode, there is no meaning in its existence.
There is only one Poneglyph in Alabasta that Cobra showed Robin. The capital was moved because of the war and destruction. So, in the end, there is no confusion, as according to the creator of One Piece that is Oda, only one Poneglyph exists in Alabasta and the anime makers just did a filler to hype things up.
I have one speculation.
In Alabasta, there are 2 Poneglyph. One was at the Tomb of the Kings and the other in the desert. Tomb of the Kings has the info about the location of Pluton according to King Cobra, but if you re-watch and re-read the arc closely, you will notice that Alabasta has two Palace. The current one and one which is destroyed due to age and the desert not being inhabitable (or some other cause).
My speculation is that the former Palace was surrounded by domes due to it being the former capital city. If I remember right, Alabasta was the only country that the King did not leave for Marijoise which is really suspicious. I'm guessing that the King was entrusted with one of the key Poneglyph that tell or is a part of the Rio Poneglyph. But since the world government we going on a hunting spree for any book or memory about the Void Century, the King decided to move his capital city to its current position and leave the other Poneglyph in the desert where no one will find it.
I think that this Poneglyph might contain some real information, not about weapons but about Void Century. Then someone from Wano must have made another Poneglyph as a decoy for Alabasta King after moving the capital while only one or two persons would have had the knowledge of the existence of that Poneglyph, making it hard for World Government to locate and destroy real treasure and after some generation of the family, the secret must have lost.