I did note that, in that post you quoted. I'd agree it's still a fluid figure with plenty of provisos and will more than likely change over the course of the parliament, but it's a terrible starting point. And it fits in with every other piece of evidence I've seen, saying that he will more likely than not win less votes than Miliband, which will be a disaster.
37% comes from this chart:
That chart says 20% for Labour voters.
There is a criticism not just of the point at which the poll was made but also the wording of it.