Another Brext lie. Get our blue passports back. It's still black, like it always was.
It's hard to tell from a simple fall in exports what impact the deal had. Maybe the fall would have been worse but for the deal.
This bit of the linked report suggests otherwise:
" In all cases but one, Japanese exports and imports of both goods and services with the UK performed worse than the equivalent flows with the EU or the Rest of the World"
The exception is goods imports from Japan. Which is bad for balance of payments and Brexit was sold as an export creation scheme.
Buuut, given that the deal was basically just the EU Japan deal with an extra bit about automotive standards and a paragraph for each of whisky and stilton, it's hard to see how the deal itself could have made much difference.
I'd suggest an alternative explanation, that the Japanese trade was at least partly trade through the uk to the EU, and it's Brexit itself that has caused the damage.