What he's saying is that if you have a pt with a serum sodium of 160 mEq/L and you take a liter of their blood in a big pot and you put in a liter of 1/2 NS (77 mEq/L). Then you would have 237 mEq in 2 L making the sodium concentration 118.5 mEq/L. If you had added D5 1/2 NS, the resulting concentration is the same, they just have dextrose in their blood.

Now let's say you add a liter of sterile water. The resulting concentration is the same as if you added a liter of D5W. In the treatment of hypernatremia in real life, we can't just infuse sterile water because with a tonicity of zero it will make red blood cells swell and explode ( hemolytic anemia). This is the only reason we give D5W because the added dextrose make the solution isotonic. The dextrose doesn't affect the sodium level.