Ironically, for much of the fight, they were the faceless horde that had to be held back by the heroic, outnumbered defenders until the cavalry arrives, and from a trope perspective there are a certain ways such horde is supposed to act. They attack when provoked (it feels like, to quote the DE leader from DoW: Soulstorm - they attack an enemy because it is there
), attack en masse in an open field when they have the advantage of choosing where and when to hit, and despite superior numbers go for the fringes instead of hitting the center from the get-go. Overall, I find it a strange depiction of how I'd imagine Eldar to fight. The ones in the Belsavis campaign reminded me of Orks sometimes
See, maybe there are good explanations for all that. Maybe the farseer flubbed his divination or just got an ominous undertone that caused him to be cautious (which ironically heralded the arrival of the space wolves and other reinforcements, which could have been preempted if the Eldar had gone for the throat). Maybe the corsairs were barely controlled at best and didn't really follow tactical orders that much, and they dragged the craftworld forces into several fights the more disciplined and experienced craftworlders would have followed differently (i.e. strafing Imperial armour columnts on an open plain). Maybe there were personal issues between the more conservative farseers (Alaitoc aren't famous for being understanding to those who have no patience for the path, and Mymeara is described as even more puritan), and the eccentric, libertarian and wild corsairs "princes". Maybe it was decided that the Phoenix Lord has first to be roused by a great battle, and it was considered an acceptable price even if a lot of the blood spilled is Eldar one. Heck, maybe there HAD to be a lot Eldar blood spilled - the phoenix lords are Khaine's prophets, and the Bloody-Handed God is not a deity that gives his boons freely even to his stepchildren. Maybe there are good reasons for what the Eldar did and didn't do - but we aren't shown any of them. We're just shown the Eldar dithering and squandering their advantages while they had full superiority, and folding fairly quickly after that. Hardly the nearly prescient, well-disciplined and well-oiled machine designed to take on forces far bigger than itself they are billed as. Heck, the DE codex mentions a kabal turning a hive world into a ghost planet, and here two craftworld expeditionary forces and a lot of corsair allies are beat back from a planet that can be described as a "backyard listening post."