What was claimed
The Treasury forecast that dropping out of the European single market will cost the economy £66 billion and reduce GDP by 7.5%.
This is confusing estimates for GDP with estimates for tax revenue. The Treasury does expect GDP to be 7.5% lower than it otherwise would have been after 15 years, if the UK relies on World Trade Organisation rules outside of the single market. But the £66 billion figure is for the resulting loss to public finances, not the economy, and it's the upper limit - £52 billion is the Treasury's best estimate.