The hype is real.
As I suspected, the passage of eons has transformed Celestia into quite a different person than she was before. Just imagine, she is now many thousands of years older than Luna. I would imagine it'd be difficult for Celestia to see her younger sister as anything more than a child compared to her, now.

It is actually quite dark, when you consider the implications. Like having a twin sibling you've spent most of your life with suddenly disappear, only to return 30 years older.

Either that, or this is the setup to a gag: she's been in hyperspace for so long, she is actually really, really horny.