>>58174443
This is reinforced by the Japanese wiki actually listing Regigigas as being one of the giants as a matter of fact on their wiki:
https://wiki.xn--rckteqa2e.com/wiki/%E3%83%97%E3%83%AC%E3%83%BC%E3%83%88
https://wiki.xn--rckteqa2e.com/wiki/%E3%83%AC%E3%82%B8%E3%82%AE%E3%82%AC%E3%82%B9
And if you go on japanese forums or 2chan, it is accepted as a matter of fact as well that the implication of the writing is implicit in stating that Regigigas was one of the original Giants, example:
https://pokemonbbs.com/post/read.cgi?no=2381693
This premise has been popular since 2012 with even a popular artist in Japan, gngmrkas, creating a series about Regigigas and Arceus' antagonism against eachother based on this.
From what I understand, the updated concept of this now that Arceus Legends introduced the normal plate is that Regigigas' giant power was put into the plate and that is why he is so weak now and or Regigigas is kind of a daisy chain entity where he was created by the giants from what was left of the eggshells and the regis were then made by him..