True, there was a three-way split to some extent, and true, the LDs got as huge boost from the TV debates .... but exactly when did they sign those tuition fee pledges in relation to all that?
But even so, even with a more even split of votes, the system still loads things firmly against the LDs. Compare their percentage of the national vote to their percentage of MPs elected, and then do it for Labour and Tory. One argument is that lots of people won't vote LD because they don't believe they can win, and they can't win because that perception prevents them getting over the line in far too many seats under first-past-the-post. After all, hypothetically, if in every constituency one party (say, Tory) got 1 vote more than Labour, who got 1 vote more than the LDs, the vote would be a dead three-way heat within the bounds of statistical confidence, but the Commons would be 100% Tory.
Given that, it's hard to see how they could have predicted the eventual outcome even though I'm sure they considered it a statistical possibility, and I'd bet they thought the likelihood of ending up in coalition with the Tories, of all people, as about as likely as finding a pot of rocking horse poop at the end of a rainbow, guarded by the tooth fairy.