Oh please. That's not a real distinction. The phone as a unit, flash plus enabling chips, is wiped in an unrecoverable way.
And the primary copy is not a backup.
This site is called Hacker News :), people might just want to learn how it works technically, so I think it is worth mentioning technical differences.
Also, it does make a small difference in practice. Erasing keys is pretty much immediate, while erasing storage can take some time (especially for phones with larger storage), so the attacker could still try to power down the device in some way to avoid all storage gets wiped.
loading story #49060771
It might be a real legal distinction, but probably not.