Why does Apple say that the iPhone 5S is more secure with Touch ID?

My friend has an iPhone 5S, and he has Touch ID turned on. On the lockscreen when I swipe to the right, the passcode screens comes up and I simply input his pass code and voila.

How is the iPhone 5S more secure than any other iPhone since it still has that 4 digit passcode system?

Without mentioning that I also replaced his fingerprints with mine since I have his passcode, so now both of our fingers work.

Thats a good point