It's not somewhat of a stretch, it's a deliberate lie. We get stories like this one every few months here on HN. It's always this kind of thing. They make up an unsupportable counterfactual and then claim a reduction relative to their fantasy world. People point it out and get downvoted by the I'm-an-EU-citizen types who can't accept that there was no impact on the UK from leaving. No negative impact on GDP, no negative impact on trade with the EU, no change to academic funding (UK is back in the Horizon programme, for better or worse, which was supposedly impossible cherry picking).

You can see all such claims are lies by just looking at the relevant graphs and comparing like with like. UK GDP has continued to track that of France. These are neighbors with similar economies, one in the EU and one out.

https://ourworldindata.org/grapher/gdp-per-capita-maddison-p...

You could also compare to prior trajectory or look at trade impact.

This study doesn't do any of those things. It makes up a convoluted economic simulation and compares against that, with "uncertainty" being a major component. It's the same technique used to predict a recession immediately following the 2016 vote that would cause 800,000 job losses. No recession happened and employment numbers hit record highs. The prior failure of economic forecasting doesn't stop them from doing it again, with full confidence.

The establishment tell these lies about Brexit for the same reason they doctor video footage of Trump. They staked their credibility on these things being a disaster, and when the sky didn't fall it shook their worldview. Letting go of their prior beliefs is hard because the updates required would affect everything, so some of them decided that maybe if they lie hard and often enough they can live in the fantasy forever.