These claims have, one must admit, been picked up and echoed by almost all British news media. It’s not just that the media have failed to subject Conservative claims to hard scrutiny, they have reported them as facts. It has been an amazing thing to watch — because every piece of this conventional narrative is completely false.

Was the last Labour government fiscally irresponsible? Britain had a modest budget deficit on the eve of the economic crisis of 2008, but as a share of G.D.P. it wasn’t very high – about the same, as it turns out, as the U.S. budget deficit at the same time. British government debt was lower, as a share of G.D.P., than it had been when Labour took office a decade earlier, and was lower than in any other major advanced economy except Canada.

It’s now sometimes claimed that the true fiscal position was much worse than the deficit numbers indicated, because the British economy was inflated by an unsustainable bubble that boosted revenues. But nobody claimed that at the time. On the contrary, independent assessments, for example by the International Monetary Fund, suggested that it might be a good idea to trim the deficit a bit, but saw no sign of a government living wildly beyond its means.

It’s true that British deficits soared after 2008, but that was a result of the crisis, not a cause. Debt is also up, but it’s still well below levels that have prevailed for much of Britain’s modern history. And there has never been any hint that investors, as opposed to politicians, were worried about Britain’s solvency: interest rates on British debt have stayed very low. This means both that the supposed fiscal crisis never created any actual economic problem, and that there was never any need for a sharp turn to austerity.

In short, the whole narrative about Labour’s culpability for the economic crisis and the urgency of austerity is nonsense. But it is nonsense that was consistently reported by British media as fact. And all of Mr. Corbyn’s rivals for Labour leadership bought fully into that conventional nonsense, in effect accepting the Conservative case that their party did a terrible job of managing the economy, which simply isn’t true. So as I said, Mr. Corbyn’s triumph isn’t that surprising given the determination of moderate Labour politicians to accept false claims about past malfeasance.