Seems to me that if the frame on that Motobecane is structurally sound that it's a screaming good deal.

I know these are not mixtes, but some lady's model Raleigh Sports bikes came in 23". I've never seen one myself, but there was a discussion here not too long ago about someone finding one in the flesh up in the Minneapolis area.

Here's something else that's not a mixte -- a 74 Raleigh Sprite that measure to be about 20.5" or 21", but look at the size of that head tube: