Oh boy, this sent me down a memory lane. I remember arguing long and hard with my math teacher in elementary school that "two times as massive" and "two times more massive than" are not the same thing, but she did not agree. That's the point where I learned that the grownup world is weird and sometimes it's easier to just smile and nod.
Considering fractions might help you understand where he's coming from. If you say something is "0.5 times as long as a one meter thing" you would expect it to be 0.5 meters, but if it's "0.5 times longer than a one meter thing", you would expect it to be 1.5 meters. The same logic applies when you ditch fractions and use integers.