As a programmer, I feel the need to point out a few things.
* "Whatever
programming language was used to make this game" is completely irrelevant. In order for a computer to do anything with a program, it needs to be compiled into machine code to make the application. Not to mention the fact that Kirito wasn't actually altering the game's "code" in any way.
* One of the few ways to alter things in a game
at runtime is if the game is pre-equipped with some kind of console that can alter the game's
resources rather than the game itself. It's not that much a stretch to assume that this "GM access console" is exactly such a console.
* Such a console would work either:
1) Through a menu, in which case, to someone familiar with computers such as Kirito, it really wouldn't be that to figure out which menu item does what and figure out how to snatch up Yui's data and store it into an in-game item.
2) Through a command line interface, in which case it's slightly more problematic as you need to know which commands do what. However, modern command consoles almost always respond to the "help" command with a list of possible commands. Familiarity with
Query Languages is the thing that helps in this case. And seeing how query languages are even more similar to one another than programming languages are, it really is not that hard to figure out.
To make all this possible in such a short amount of time merely by randomly mashing the keyboard, that's
Hollywood Hacking.