More than likely it meant forever, because he says "forever" after that. Kinda like 70 times 7 wasn't a literal amount.

Ruth was likely a Moabite by country only not by blood. Kinda like one may be referred to as American because they live here, rather than were necessarily of our stock.

I don't know enough about all their bloodlines to assert it's I am right but it would make it more consistent with his word.