There are examples of MTG cards referring to a single opponent in old editions[1], digital editions[2], and Un-sets[3].
[1]: https://scryfall.com/card/leg/6/clergy-of-the-holy-nimbus
[1]: https://scryfall.com/card/leg/6/clergy-of-the-holy-nimbus
"Chandra's Outrage deals 4 damage to target creature and 2 damage to that creature's controller."