I really think that what he is trying to say is that the pound going down may not end up being a catastrophic event as some are predicting, and the drop will be nearly negated by increase in volume. His personal business is lumped in with the rest of them, so I'm not understanding how you think he's saying it will be a windfall for him if you're also saying it's a crisis for the other businesses. They're all in the same boat. I think he's saying they will all be OK.
I personally think the markets will eventually stabilize and the UK will be better off for the Brexit vote in the long-term.
The specifics of how the markets may or may not react is irrelevant and beside the point that Trump was quick to make it about how it would benefit his personal business in the UK, rather than, say the people of the UK or the people of America.
This wouldn't have been a hard concept for you to grasp 4 years ago, but Trump has made disturbingly unpresidential behavior "the new norm".