I encourage others here to read the study and analyze its methodology before outright accepting the conclusions of a self-described “think-tank devoted to making the EU work better, and strengthening its role in the world“.
One of the several issues is with how the “dopplegängers” are selected. A basket of (weighted) countries is selected with economic structures and pre-Brexit performance similar to the UK. I haven’t analyzed the linked Python code and datasets, but the text gives scant information on what exactly makes a specific country similar or dissimilar to the UK.
The rare exception is when it compares UK’s GDP growth with Australia’s and New Zealand’s instead of Italy’s, when it says it ignored the latter because of its “rapidly ageing” population.
In the “Services trade” rubric, UK actually outperforms the dopplegänger, until the author removes the country with the largest weight, at which point the UK’s advantage disappears. This could be interpreted as some questionable data massaging.
None of which is to say that I necessarily think Brexit was, or will be, an economic success for the UK. This is not something that can be easily determined at this point, especially with Covid. But these studies should be taken with a grain of salt.