On other SE sites, this should be a comment on the answer and the person who posted the answer would choose to edit their answer to include it or not.
On other SE sites, the comment will live longer. If it were Stack Overflow, where there is no time for mods to comb through posts and delete comments, I'd say post it as a comment to the answer. It will live forever and be helpful that way.
Here, this comment will vanish fast. I've had comments vanish over the time I needed to go out and grab a pizza from the store, doubting my memory of posting it in the first place when I came back and it was nowhere to be seen.
So I guess it's a matter of how much of it is an answer and how much is just agreeing.
I sometimes post disclaimers saying that my answer is not exclusive and that while I think it's valid on it's own, it does not mean you couldn't or shouldn't do $OtherAnswer as well. I think that's valid. Example for "not exclusive".
I sometimes post answers that come to the same conclusion as existing answers, but for a completely different reason or using a completely different rule. I do mention the other answers to signal that I don't disagree with them, just take another reasoning to the same end. I think that's valid as well, because while one answer with that conclusion might have been enough, the other answer might get invalidated by time, new editions, comments on how their reasoning is wrong or they might just get deleted (could not come up with an example on short notice, this is more the case in other SEs).
It rarely happens, but sometimes, the obvious, correct answer is not the answer. Personally, I think quoting with a reference is the right thing to do, as it keeps the answer from being incomplete should the other get deleted. Example of referencing another answer.
So what I want to say is: there is no single answer. Check the post. If it's good information, keep it. If it's just a one-liner or no real information when you take away the "I agree with the other answer" part, then flag it as low quality. Because it is.