Those were never explicitly stated to be the Populares, who wouldn't have any reason to be occupying the royal palace even if they hadn't been mostly wiped out by the attack in the Burn.
Considering that Nerva's forces seemed to have the upper hand in the war unfolding in the capital, it's more probable they were his soldiers.
For that matter, I'm still really curious about what role he has to play in this whole mess, though considering that there are organized Garlean forces amid the tempered, he might simply be there to provide some semblance of order amid the chaos. I don't doubt he'll probably be an obstacle in 6.0 tempered or otherwise, though.