BM say that they are failed Ancient Giants, but she didnt put Kaido in the same boat with them. Pehaps Kaido is a Dragon and have a Ancient Giant fruit or something like this.
And i think the reason that Kaido have chose Onigashima as his home is because of the skull of the giant, who i belive have in the Void Century moved islands and created Wano in the way it is today to protect them from the WG.
Seems that the Ancient Giants will have a bigger role in the lore than we think, could even be that they are the ones that have pulled the islands to form the grand line.
The Ancient Weapons have a lot of connection with the geography of the OP world and have the power to destroy the system of Red Line, Calm Belts and Grand Line that seems to be a creation of the WG as a way for them to have control over the world. Pehaps the Giants will have some connection with Uranus or Zunesha.