I think the message was real. Probably, there was no distress signal from the 5 fleet because they were more than likely being jammed by the Supervision-Protodevilin Fleet. (as Geperunichi uses Supervision Army ships, human fighters and soldiers, I would think they have something to royally screw any chance of calling for help).
I think it wasn't a bad idea to land and I bet they did send out scouts (they more than likely knew about the dinosaur things, but in the 7 Fleet's case, they didn't have time to. <- my explaination for why no one warned Basara, Mylene or Gamlin about the local life). They probably realized there was no danger in the water or the coast, so they landed.
I doubt they took down the WHOLE 5 fleet though. I do think some warships stayed in orbit.