" have claimed (correctly) that I read economics at university "
Thats an appeal to authority because it suggests that you are some kind of export.
"You obviously didn't like it when I pointed that out."
You didn't, you also failed to note that Intra EU trade was also growing.
"25 out of 28 EU countries are doing better at trading with non-EU countries that those in the single market."
This is not what the document said, you are incorrectly assessing it. What the document shows, and states, is that the ratio of trade with other EU member states to non EU member states decreased.
The document, if you had read and understood it, actually shows that EU inter trade has increased, but the ratio has decreased. Which means that trade outside of the EU is growing at a faster rate, but this is to be expected because of convergence. In fact what the document shows is that all countries other than the United Kingdom, exported more goods to partners within the EU than outside the EU. Which is backed up as 44% of UK exports go to EU countries.
However not commented on is how of those exports to other countries have increased because of EU FTAs. 15% of the UK's trade outside of the EU is based on EU FTAs.
So no, I didn't read it incorrectly, you did.