It's pretty clear that the Romulan military and the Romulan government are blood enemies (goes with being Vulcanoids, I guess). Using Romulan military might against the Romulan government seems a given, then - but doing it by proxy sits better with the majority or empowered minority that needs to be convinced to support the winner. So staging a Reman revolt and secretly giving the designated Spartacus some decisive weaponry seems plausible.
But Shinzon is a lying scumbag. Just about the only thing likely to be true in the tirade he gives to Picard is that the Scimitar was built in a secret base. I mean, what sort of Romulans would build ships in public bases? And of course building is for slaves.
This is hardly a plot hole, and not even much of a plot oddity. The gaping hole in the plot as given is the motivation of the main villain. We're given none, and while we're free to think up one or several, it's not the sort of gap-filling the audience is normally tasked with in a story - it is the story. Not only do we fail to learn what makes the villain tick, we also fail to see the villain tick. Instead, he just postpones! Gimme the twists and turns of ST6:TUC any day instead.
Timo Saloniemi