Frankly it reads as nonsense to me and not just because I disagree with it.This one of the best articles that I have read on Brexit
https://www.nytimes.com/2017/07/28/opinion/dunkirk-christopher-nolan-brexit.html
It suggests an ambition or result of independence and separatism that nobody is suggesting. Brexit is about more trade, not less.
We don't need to have the best manufacturing sector, we can buy from India and China (and still Germany too). If we don't have the employees we can still import them, it's just that now we don't have to pay them or their families any in or out of work benefits.
And whilst 45% of all our exports sounds like a big number (I suspect it's chosen as the figure of choice for that reason), 14% of our GDP doesn't. In fact, it's not hard to imagine that savings could be made on that 86% of GDP that could cover the cost of even a massive drop in EU trade causing a 1-2% drop in GDP. That's without the benefits of reduced input costs and a larger market to trade with.