The author said the average length of cohabitation is 18 months. If they'd been married instead of cohabitating, that would mean they'd be divorced instead of simply going their own ways. Even though the odds of married people splitting is less than those cohabitating, that still means in those 18 months there would be divorces instead of simple splits.
Yes, but they weren't married. That's the whole point.
Your thesis doesn't hold water. In the past, where cohabitation was almost unheard of the divorce rate was in the single digits.