Thank you so much for your insightful comment, @iskawrites. The topic of time traveling is mind-vexing.
Elena being able to rewrite his code, made them the power couple.
Elena actually did not rewrite the code. She just passed on the message that came from the other Ian on the phone. So, the story implies that Ian from the future sent the code to Ian from the present.
Now, you left me wondering what they did with the device they made powerful by coming together. Did they change the world with it?
I was thinking about this too. In my mind, they are about to embark in a grand time-traveling adventure, like lovers in dangerous times.