Are scripts "open source" by definition?

The other day, I tweaked a script for a friend's World of Warcraft addon. He was surprised that you could edit the addons—that they were "open source." (Word of Warcraft addons are written in the Lua scripting language) I found myself wanting to say "Sure you can—all scripts are 'open source'."

Is that true? Sure, some scripts can be compiled to bytecode, but aren't almost all scripts interpreted? That is to say, doesn't the device interpreting the script need the "source," by definition?