The clearly destroyed buildings in the background.. You know, lots and lots of twisted metal support poles
Why were the bodies left?
They often are if they are hard to reach or you have more pressing buisness. In a battle for survival of the species you won't spend men and resources to dig graves when the enemy is about to blow up your home planet.
Even after that, there would be far more pressing issues - the rebuilding of the shatterd worlds.
Deneb IV (it was that, right?) looks uninhabited in Fs2 time, prolly nobody went back there.
Many dead soldiers trought the history were never burried properly, but were just left to rot.
The battle of Deneb in the cutscene was not the batlte with the Galatea (altough that also took place in Deneb).
We can see a tow fleets colliding - there are various cruisers and destroyers from all races. Something the FS1 engine couldn't really handle.
As to how the soldiers got there...
Falling out from a ship? Impossible, there are only soldiers there, armed to the teeth.. unless all of them fell out with their weapons.
Battle with the Shivans? Possible, but where are the shivan bodies? We know terrans are capable of taking thenm out, espcially with heavier weaponry. And I doubt the bodies would be so "neat" - they look like they were killed by bullets/laser fire. A shivan would slice them in two or fry them.
Lucifer bombardment? Being hit by the lucifers beam would vaporize the bodies. tehy could have been away from the city and killed by the blast that hurled them far away..But then again, there are no civilian bodies that would also be hurled in that situation.
The whole custscene talks about the past and ghosts that haunt the ruins (the GTA being a ruin in some way too). Three great threats emerged in the FS past - the shivans, the HOL and the GTI rebellion.
So the cutscene references them.
I still think those are the ruins of some other battle and not a batlte with the shivans.
If they bombarded the planet, why send ground troops for that useless peace of rock? A few surviving humans are no threat or imeddiate concern of the shivans - tehy would die anyway without help.
HOWEVER, givan that this IS a cutscene that does appear to hold some conflict with in-game cannon (like the Hades..IF it is the Hades in the first place..It might be some older GTA destroyer) it should probably be tied more to hte shivans than anything else.