Yeah, I don’t get it either.
It’s a good trade policy, but it’s a very twisted path to say it’s paying for the wall.
Almost all trade revenue is between individuals be they be persons or companies. There is no significant sovereign revenue gained or lost in international trade.
So, with this change, Mexican companies will bear the brunt unless they choose to end the loophole trade altogether. Since it is rather akward, that is what they will do.
There is no resultant sovereign revenue stream to pay for the wall.