As I edited, the reality is that for anyone, boy or girl, puberty varies. That's obvious. But in comics, where the illustrator controls how their characters look, I would just find it odd to give a pre-teen girl breasts.
In any case, I understand your argument about teen Jon being superfluous. I even understand the argument that just doing it for a relationship would be thin motivation. I'm not arguing what works best (everyone knows how I feel there). I'm just saying the tea leaves to me suggest teen Jon in the main verse, kid Jon in digital adventures with Damian. Another motivation for doing this could potentially be that moving forward they don't want Superman himself to be constrained to a Rebirth family dynamic in his main comics. Again maybe something they figure they can do once a while in a separate digital space. I don't know. Like you said, we'll see, perhaps today.