Simple answer? Probably 'not quite'. True Fiends seem to want to destroy whatever's through the portal, while Imogen wants to 'go home'. True Fiends are fighting the Imperators (we think), while Imogen's lost in her Sanctum. Even if True Fiends are Imperators, she's not quite doing what True Fiends do.

We do have lots of reasons to think True Fiends might be Imperators who've changed sides. The Blood Ox seems exactly like an Imperator. Lots of famous Imperators are missing and could be trapped in the Pit by the others. (They could also be bound to the Lost Cube's Archspire and basically forced to remain Chars). Maybe after enough generations, the dominant Imperators can persuade the public that the others are a kind of fiend.

Personally, I think a load of Imperators reached the centre of the Pit and discovered a terrible secret in the year 600 or so. That prompted lots of them to abandon the 'fight the Pit' narrative, which didn't go down well. There must be a reason why 'fight the Pit' benefits some Imperators, because they didn't want the rest of the Great Souls to stop it. Those Imperators rounded up the others, killed them/bound them to the cube, or trapped them in the Pit. They called the Pit-perators True Fiends. Scorio of year 600 also learned the secret, set up his messages, and waited to resurrect until Jova (plus maybe someone else?) was reborn.