Economists say that trade can make everyone better off.
This means that trade can make everyone worse off.
Consider the following two statements:
1. Trade can make everyone better off.
2. Trade makes everyone better off.
The second statement is clearly stronger, yet economists don't choose this statement. Why? Because statement two must be false! Then it's just a hop skip and a jump from Trade can make everyone better off -> trade can make some people better off -> trade can make everyone worse off.
Your grasp of what economists say and argue is tenuous, at best.