This is one reason users are encouraged to ask only one question per question, but sometimes that line can get fuzzy.
Let's assume, though, for a moment, that there's a question where it doesn't make sense to post two questions. Let's also assume this question has an easy part, and a hard part. I would say that if you're only answering the easy part of the question, perhaps it would be best to leave a comment, instead of writing an answer. But if you're answering the hard part, it's okay to leave an answer and leave the easy part unaddressed.
This depends on the situation, though. If the easy part is only easy for you because of some kind of expertise you have (so it's unlikely to be easy for the rest of the community), I might leave that as an answer, but I'd probably explicitly mention that I'm not so sure about the other part of the question.
Fact is, some questions will be difficult to answer with a single answer. If your answer can stand alone amidst a group of answers, and add something helpful for the O.P. and interesting to the community, go ahead and leave it as an answer. On the other hand, if you are merely stating the obvious, you might want to use a comment instead.
Moderators can turn answers into comments. In fact, other users can flag an answer when they believe that's what should be done.
In the specific instance you asked about, the O.P. is relatively new, and is feeling their way around ELL. I wouldn't be surprised if that question gets modified some. If there's any fault there, I think it rests more with the question than with your answer.
Thanks for asking about the matter here.