Nah, California and Manhattan are mentioned in RDR. Different universes, just a fun easter egg
They are very clearly in different universes, we've got several things to indicate that. Having things in GTA Online as a marketing ploy does not make them in the same universe, buddy. Maybe you should learn your lessons.
So in your opinion , because they decided to advertise this event , it makes any lore shown irrelevant ? Both aren't mutually exclusive.
We first receive an email from a certain Vanderlinde.
he's the antagonist from RDR1. During RDR1 , we learned that our protagonist Jhon Marston was running a gang with Vanderlinde in their youth.
It's pretty much confirmed that at some point during RDR2 , Dutch will play a major role again. The quest itself reveals many details.
We first find a note referring to a certain family and that they were fighting for who would keep these pistols. It apparently went badly enough for them cause' one ran off with the treasure , obviously in bad therms.
Then we find a bloodied shovel. Then we find a box for a left dual pistol , empty. Then we are directed to a naked body in a cave.
This dead body got a white sock and a red sock , his boots seems to be made of old leather. ( could hint that he came from the past but we can't see his clothes ... someone removed them after killing him ... ) His right hand is possibly showing a 6 or a 9 , or doing the hand sign A-OK.
Then we finally find the treasure. There's 2 hill billy bodies with modern cloths and modern weapons next of them. This could hint that those 2 guys heard the noises of someone digging and went to rob this person , only to both meet their death. The treasure is dug up but closed , the shovel was left there planted in the soil. When we open the wooden box , we find the golden engraved right dual pistol in it.
On the pistol we can see an armory stamp (DDP arms co.). This is a clue i believe. We can also find the name of the transport company on the box (Boles Overland) , the number (7) and the fact that it comes from a stage coach.
It's important to note that we can see one of the gang members in the RDR2 trailer wielding dual pistols. After this quest , we never learn who killed these 3 people and who kept the left dual pistol.
That makes quite a lot of details for just a '' marketing ploy '' . You're free to believe what you want though , just like me.