From the article:
"Herndon-Ash-Pollin find that they exclude Australia (1946-1950), New Zealand (1946-1949), and Canada (1946-1950). This has consequences, as these countries have high-debt and solid growth. Canada had debt-to-GDP over 90 percent during this period and 3 percent growth. New Zealand had a debt/GDP over 90 percent from 1946-1951. If you use the average growth rate across all those years it is 2.58 percent. If you only use the last year, as Reinhart-Rogoff does, it has a growth rate of -7.6 percent. That's a big difference, especially considering how they weigh the countries."
Excluding the post-war growth years of those nations which were geographically isolated from the bulk of the war makes some sense, but failing to mention why that was done does not. These British territories experienced high levels of growth, presumably because their intact infrastructure was used to augment the growth of Britain, most of whose infrastructure was in tatters. It could be said they were used as satellites for Britain's industrial regrowth in the post-war years.
There was an article, which I think I linked, a while back which argued that countries such as America, Canada, and Australia enjoyed post-war growth, not because of their grand know-how or any other particular expertise, but rather because most of the infrastructure of the other powerhouse nations were bombed into oblivion. As those nations, with centuries of know-how and experience, regained their infrastructure, they began to supplant the upstart nations in growth, which local economists lamented as the 'decline of America', the 'decline of Australia', etc.
America's post-war rise, like Canada's and Australia's, was a result of geography. Excluding those years as 'anomalies' would be the equivalent of the oft-cited 'hide the decline' of glabal warming lore. It may be justifiable, but it should be up to Reinhart-Rogogg to justify it.
Quod scripsi, scripsi.