Thomas de Carneillon actually has an Extremist's face (red guards) and an Extremist's voice. You can hear it if you start running away from him. I'll have a video of this amusing "attacking bodies" glitch uploaded soon enough, then you can see it for yourselves. It happens after he throws his flashbomb and you chase him to the second part of the fight.
This is actually really helpfull, I was pondering whether or not I should replay the prologue to see if there's room for some fun but seeing as how limited the gameplay is, I think I'll pass.
Still a shame that you are unable to play the prologue without erasing your savefile and starting over.
Having a little exploring on my pc:
Looks like in prologue the game only loads a very small scale of Paris. It's possible to explore Paris using a teleport cheat on pc.