Sure we see Dutch's gang in the trailer, but it's all for hype. Dutch's gang will be in RDR2 but they will just be a mission contact like West Dickens, Irish, Bonnie MacFarlane, etc. Dutch and his gang will be a mission contact who we work with for a few missions. Arthur has his own gang and probably doesn't interact with John Marston much (if at all). This is why John Marston never mentioned Arthur.
Rockstar knew presenting the trailer this way would create hype and raise questions and produce theories, etc. In the end, Arthur is NOT apart of Dutch's gang, and Dutch's gang is not who we see riding with Arthur is the trailers. We see Dutch's gang in some RDR2 images and a few times during the trailer, but these are small encounters of a much bigger story in RDR2.
Believe me or not, I don't care lol. This is how it is though.