Ok, first of all, no and i mean no Eldar would sacrifice his craftworld so save some monkeigh!
From the "Doom of the Eldar" Rulebook (yes Irandrura, i know you already love that one
):
"The first warning the Eldar of Iyanden had of their fate was brought by the craftworlds far ranging scouts... Already a dozen Imperial planets had been consumed... and, although the Imperium was mounting furios counter attacks as and when it could, it would be months until a major Imperial taskforce could be mobilised to deal with the threat. By then Iyanden would have been overrun."
We learn, that farseers are not all knowing. Perhaps the warpshadow of the fleet did hide it as well from psykers. Craftworlds are vessels that can only travel the biggest of the webway tunnels and a slow otherwise. Its totally reasonable, that there was no escape route within reach.
Farseer Kelmon then called in the council, that proposed sevaral things:
- shilding the craftworld behind a powerful psychic shield, to avoid contact
- attacking the nids with the fleet before they reach the craftworld
"Both courses of action were deeply flawed however, because they took no account of the sheer size of the tyranid swarm. It took a powerful speech from the scout Irilith, who had seen the Hive Fleet at first hand and understood the terrible threat that it represented. For over an hour she drove home to all present, that the Hive Fleet was to large to hide from, or for the Eldar fleet to defeat of its own. It would take the combined efforts of every Eldar on Iyanden Craftworld to have any chance to turn back the Tyranids. Even then they might not succeed to defeat the alien menace."
So here we see, that the Eldar could obvisously not run, since that was not even proposed, hide, or let the fleet deal with it. It was a battle of sheer survival, that would have very gladly been avoided.
In the next lines the preparations of the Craftworld are described, recalling the entire fleet, arming every citizen, fortifiying the entire craftworld, awake the Avatar, and the most terrible: awake the ghostwarriors (wraithguards now) (Something they would have never ever done without dire need).
They could not run, so they had to fight. They would have sacreficed a thousand imperial worlds without a second thought to avoid this fight.