So this is one character in one dialogue, already requiring a script.
Now do the maths: the game will probably at least have 6-8 possible companions and hundreds of unique dialogues.
Calling it a script is overselling it. It's basically a single IF or switch statement. And your solution already means the game would be testing if Sebille is present anyway. You just need to check that she is present as a companion, and then you can force the response without player input. Generally, companion personalities are fixed enough that their interjections can be predefined.
If they build this into the dialogue system, then it's practically just a case of ticking a box.
