I always find it difficult to differentiate between a comment and an answer when I feel like the question can be "answered" in one line. But I'm generally reluctant to post an answer because I feel like I'm not being detailed enough. If you look at blackcompe's answer here, it is technically correct as it stands. But should it have more detail to actually qualify as an answer and not a comment?
If your proposed response can stand alone and actually answers the question, then it's perfectly okay to post it as an answer. Otherwise, you should post it as a comment.
Of course, obviously the better answers will generally be longer than one line. To bulk up the content, you might consider doing things like:
I will frequently post a comment instead of an answer when I'm either guessing at a solution, providing only a hint or a link, or otherwise unable/unwilling to write a complete answer. Ideally, someone else (including, potentially, the asker) will come along and expand my suggestion into a full answer.
I think the policy of demoting simple answers to comments is hiding good, terse, answers in the comments section. I know at least I skip over them, because they rarely have good information.
So to answer your question: Yes, they need to be more complete to get past the moderators, even though they are technically fine without the unnecessary explanations.