I looked up in a dictionary for “might just as well” and I found the following definitions:
might (just) as well:
1-used to that someone should do something, because there is no good reason to do anything else
2-used to say that the effect of an action or situation is the same as if it was another one
Which one is intended here?
“Why do you keep talking to my horse instead of to me?” asked the girl.
“Excuse me, Tarkheena,” said Bree (with just the slightest backward tilt of his ears), “but that’s Calormene talk. We’re free Narnians, Hwin and I, and I suppose, if you’re running away to Narnia, you want to be one too. In that case Hwin isn’t your horse any longer. One might just as well say you’re her human.”
The girl opened her mouth to speak and then stopped. Obviously she had not quite seen it in that light before.