Read on to the end of the article:
"The CER used a statistical model that compared the U.K.’s economic performance against predicted output if the referendum result had gone the other way. It did so by identifying which OECD countries’ gross domestic product, consumption and investment data best replicated the U.K. economy in the two decades leading up to the referendum."
The CER article is based on comparisons with what it believes would have been the situation had we not decided to leave the EU.
Total speculation.