DK is referring to when he was sealed in Purgatory at the end of the Holy War.
Time passes significantly faster compared to the normal world (
1 minute = 1 year in Purgatory), thusly 100 million years in Purgatory would actually be around 190 years.
In truth, DK should've said 1.6 billion years, since he's been in there for about 3,000 years from the perspective of everyone in the normal world.
OG Demon was likely created after the Demon World, especially considering that it
was there before he was split into Cusack and Chandler (who are listed as being the same age in the same databook).