isanswered: yes/true/1 returns only questions that have at least one accepted or positively-scored answer; no/false/0 returns only questions with no accepted or positively-scored answers.
isanswered:0 operator returns both questions and answers.
Assuming that this is the intended behavior (altough I'd find it weird regardless) the problem might just be with the description within the Help Center, which should have been [...] only questions or answers to questions [...].
Either I'm missing something or this is not the intended behavior.
EDIT: this has now partially been fixed by changing the description in the help center, but the operator still returns both questions and answers.