Complete copy of a post just 10 above your original question:
"At the moment, Public story is always normal difficulty. If you want to play on Advanced or Normal-Hardcore or Advanced-Hardcore, youāll need to start a private match.
We donāt want to split the matchmaking pools for public more than we have to, so we think this is a good way to start. If we get enough requests, we can make a Public pool for Advanced, or the hardcore variants."
-Jythri (Creative Director for Battleborn)
Note the second paragraph.
Conclusion: You will get your harder difficulty queue sooner or later. And in the scenario that the game flops, youāll have difficulties filling a lobby anyways. Basically itās about playercount.
Also from other sources that I wonāt search out now:
You donāt need to have 5 people to start a story mission. The difficulty adjusts to your player count.

But honestly this is battleborn, I donāt think they will go that mmo route.