They did? I must have missed that one. But I do think it's an interesting POD.
As I said above, and basing this purely upon my own memory of the time, only a week or two after Labour were riding high in the polls and Brown was being urged to call an election and then being accused of weakness and indecision for not doing so, Cameron gave that very well-received speech and unveiled some Middle England-friendly policy points and suddenly surged ahead. It was like a mini version of "Cleggmania", iirc. I agree that a much, much-reduced Labour majority looks like the most likely outcome, but if Cameron could recreate a similar surge during a General Election campaign, I wouldn't rule out the Conservatives just edging it. Or indeed a hung Parliament. But I do agree that just about any of the likely outcomes look disastrous for the country, and indeed for Labour, in light of what we know happened a year or so later.
And yes, the Lib Dems were on course to do quite badly in a general election at that time, I think (worse than 2010 probably), compared to their really pretty good performances in the prior ones.