Slitherine

I noticed in game that Libran's population is listed as 2,102,656,289 (2.1 billion), but according to the map from Quantam Mechanix, the population from Libran is 2.1 million. This makes sense since it's supposed to be a jungle planet that was settled later. Just pointing it out....

We don't know if this star map is entirely canonical.
It could just be a whif in the numbers, can happen.
But it could also be that libran suffered extensive damage in the first war, we are currently in.
The later colonization shouldn't be a reason for fewer citizens because it still is a time span of around 2000 years and with the population explosion of the last 200 years on earth from a mere million to around 8 billion, it shouldn't be unrealistic to have around 2 billion inhabitants.

Well, the planet is also described as a jungle planet that is largely a nature preserve. Granted, this doesn't mean anything, like you say, unless the star chart is canonical. But all the other populations in this thing seem to match those in the game, so I'm guessing the developers used this as part of their world-building.