I got the impression that the planet had 'settled' down again after the civil war. It was devastated, but capable of sustaining life, and government. I'd have to watch the episode again to be sure though.
Having looked at the relevant passages again, it's not entirely clear what the situation is. However, there are some points to keep in mind:
-apparently, the worst of the civil war may have been already decades before the Protostar emerged ("decades later, hope returned"), leaving potentially room for some kind of limited restoration. Then again, the civil war and the road towards it also was said to be decades, so it may be less than 20 years if Ascencia was exagerating a bit.
-when Chakotay manages to break out temporarily to send the Protostar back, he appears to be at the ruins of a big city, still in a state of massive disrepair
-I'm not sure, but when the Protostar is shown breaking into orbit of Solum, the surface seems to contain many very large craters, possibly indicative of a very serious surface bombardment. We don't have a pre-civil war state of Solum (seen from orbit) to compare to, though
-"The order" is only formed after the Protostar left prematurely, so it seems there was already more or less a shared purpose before that (at least among those who still had some power and would later form the order)
-they sent their last 100 ships, each manned by one person and one Drednok (and presumably, a number of lesser droids like the Watchers we see at Tars Lamora)
-both The Diviner and Ascencia seem to consider their society to be destroyed, Solum ruined and their species as good as gone. The Diviner underlines this by considering Gwyn to be the last of the Vau N'akat after he would die, unless Starfleet is destroyed (and though this is obviously not literally true).
The penultimate point I think does point to there being few survivors. The decision to send all 100 remaining ships - which are probably very important for the survival of the remaining Vau N'akat, given they offer shelters, replicators etc, while the cities on the surface at least seem to be totally decimated combined with placing only one (wo)man on each of them is difficult to explain otherwise.
If they had plenty of survivors, surely they could have easily sent a decent crew on each (which would have been very helpful, and avoided the need for Gwyn, for example), unless there are other survivors who don't agree with The Order and the numbers of The Order are small (in which case they would still not be united). And if they believed Solum could be rebuild (the society and some form of infrastructure), the ships would have been very helpful in their own time and they may have hedged their bets by sending, say, half rather than all of them. Instead, they went all out to change the past.
My impression is that the survivors were likely mostly sustained by their advanced vessels (who may well have been the primary means of the destruction in the first place), and there wasn't much else left on what would seem to be post-apocalyptic Solum.
This does assume, of course, that Ascencia's statements are more or less accurate. They are at least partially confirmed by the Diviner, however.
