I have been unable to find a definitive answer to the question of whether format-shifting (e.g. ripping your own CDs so you can listen to them on your smartphone) is currently technically legal in the U.S. or not. But “everybody” does it.
I would imagine that if you ripped CDs that you checked out from the library and deleted the files when you returned the CDs that this would fall into the same category, both legally and ethically, as ripping CDs that you own; but I do not know for sure.
The article that you linked to doesn’t want me to read it without registering, but it seems to be mainly warning people that they’re not allowed to make copies of library media to keep.