It is an implied targeting, as it finishes the sentence that has already referred to characters with resistance. Saying "...heal each character who has resistance 5 or more and exert each character with resistance 4 or less." is not an efficient use of space. Decipher only had so much space on each card for its game text, and sometimes they needed to ask players to use their heads.
Neekerbreekers' Bog only affects characters with resistance, whether it heals them or exerts them, and never affects the Shadow Player's cards.