You are correctly trying to say it’s well defined, but you are complaining about the wrong comment. You should check the meaning of “arbitrary” again.
Anyway, it’s not entirely arbitrary because it was created to represent a “round” fraction of the Earth’s circumference that is similar to the length of a person’s arms. But it deviated from that too, so it’s subjective how much that counts.
You are correctly trying to say it’s well defined, but you are complaining about the wrong comment. You should check the meaning of “arbitrary” again.
Anyway, it’s not entirely arbitrary because it was created to represent a “round” fraction of the Earth’s circumference that is similar to the length of a person’s arms. But it deviated from that too, so it’s subjective how much that counts.