I hope that when all this is over, we don't find out that this was the catalyst for Yuzu's self-realization, I'd rather it have been earlier.
Feels like it's... kinda between the two. We don't know what Yuzu was thinking precisely, but she seemed to be convinced that she could just keep the status quo forever. It's hard to know if it's because she was content with it and didn't think any further, or because she was worried about taking risks in changing.
I mean, I don't buy that she viewed Minami as just a friend. Especially as we've seen her act as a normal friend with the others. So she must have had some kind of "realization" long before, without pursuing that line of thought all the way. After all, she reacted immediately when the kouhai asked if she'd be fine with Minami dating someone else, without any hesitation or denial.
Realizing that Minami might be courted by someone else probably forced her to put certain words on her feelings that she might have avoided before. To me, it feels less like a realization of her feelings and more a realization about what they mean in terms of the future of their relationship.
Either way, that was a surprisingly straightforward resolution of the kouhai plot. At least one of them three is capable of expressing her thoughts and feelings.