I don't know man, there's so many new questions. I don't think we're going to get the whole 'fragmented empire' thing as we once originally thought. The Empire looks a lot more Sith-ish and that new star destroyer looks like one of those Harrower Class Dreadnoughts or whatever they're called.
The planet they're on is Jaku apparently, not Tatooine as originally thought.
Im not so sure about the fragmented Empire, in the one scene where the Tie-Fighter shoots at the Stormtroopers in the Tie Hangar i had the impression as if some of the Stormtroopers in the Background were attacking the ones in the front.
But that was only my impression of this 1 second bit.
And with the Star Destroyer i looked at every Star Destroyer in the Legends-Canon
didn't find any fit, so its something completly, but yeah it does share some similarities with the Harrower Class Dreadnought and the Centurion Class Battlecruiser, both Sith Vessels
which would support your idea that this Empire is a Sith Empire.
Beside that i just had an idea:
As i said:
It seems, that Stormtroopers are attacking stormtroopers, in the scene where the "Chrome" Tie Fighter is in the Hangar.
And there is a Scene where you see Boyega in an Airshaft, and i think we now know, that he is a Stormtrooper.
So what if:
He is a Stormtrooper of an Imperial Faction, in the Airduct of a Stardestroyer doing some shit
the new Sith Empire comes on board for Negotiations to get the Empire back together, but instead of negotiating they start shooting
and Boyega sees everything, while he is in the Airshaft
after his faction is destroyed he flees to the Yaku