The history of the Reef starts in Destiny, and progresses through The Taken King, and the rise of Oryx. In this expansion Oryx destroys the Awoken fleet in the Reef, and presumably kills Mara Sov, but we see Uldren Sov escape the blast.
Destiny | The Taken King | Introduction cinematic
However, it can also be assumed that
Uldren Sov is still on Mars
and
If you stand near Executor Hideo, there is a random voice line you can hear from one of the unnamed npcs talking about how he knows his sister is alive at the reef and he wants the guardian to go find her
If you inspect Telesto, it's flavour text is
Vestiges of the Queen's Harbingers yet linger among Saturn's moons.
So this could be taken as some of the Awoken loyal to the Queen did escape Oryx and Ghaul.
It's worth noting that The Reef wasn't destroyed by Ghaul and the Red Legion during the Red War campaign either, but we're not told about this in the campaign of Destiny 2.
Some NPC's in Destiny 2 talk about being able to reach The Reef, so presumably since the Queen's absent, Guardians are not welcome in The Reef any more.
My take on it, having played only the first Forsaken mission.
To me this makes sense. As Uldren Sov will blame the Guardians for drawing Oryx to the system after the death of his son Crota, and causing the destruction of the Awoken fleet. With his sister out of the way, he's free to claim ownership of The Reef. I think he breaks the barons out of the Prison of Elders to build his army.
The Vanguard also state to you that they will not go to war with Uldren Sov, and if you do, you are on your own.
So to answer the question, I don't believe that he was. I think he went in to break out the barons.
Ninja edit
I found some more reference material for those who like reading.
- The Maraid - The reef wars
- Mara Sov
- Uldren Sov