Wait a minute, I just realised something
Did Moon marry River when she was 19?!
Moon's first entry on her Butterfly takes place right after her news on her marriage. And, according to herself in "My New Wand", she didn't manage to dip down until she was 19, probably due to Glossarick's long mourning period after the passing of Queen Comet.
Besides, Moon has clearly aged between her wedding and her Butterfly, further proof of this would be comparing all of Moon's younger selves and her current self.
Just Married! Moon resembles teenage Moon a lot more than Current!Moon, who, again, according to the book, had just turned 17 by the time she became queen, making it a two year gap between her coronation and her wedding, if I'm right on this one. Whereas Butterfly! Moon (both the drawing from the book and the screenshot) is clearly modeled after Current! Moon's design.
On a side note, let's not forget dipping down isn't just achieving one's Ultimate Magic Form, but, as seen in "My New Wand" and "Meteora's lesson", the mere ability to use magic without a wand. Which makes it all the more possible for Moon to have married River young.