The IF is set in 192 AD, which is mentioned briefly at the beginning of it. Hati focusing on the Romans' loss over their victory is probably just because they hate Romans. Their intense hatred of the Romans is why they have no tolerance for the Britons for allowing themselves to be conquered and Romanised by them. Remember, Hati doesn't see the Britons that fought Boudicca when they look at them, they see the Britons that joined the Roman military willingly.
The Legion is not explicitly stated, but the IF takes place in Mogontiacum. The legion that was stationed there at the time was XXII Primigenia. Similarly, the Emperor who's name has thus far gone mysteriously unmentioned is certainly Commodus.