That's an auto-prompt from the SE engine, which apparently is not smart enough to detect your comment upvote. (I guess when you vote it checks for a comment from you.) If I recall correctly, the prompt is phrased as a suggestion. In this case you already took a reasonable action, so don't worry about it.
Every now and then somebody proposes (across the network, I mean, not here specifically) that comments be required for downvotes. I'm pretty confident in predicting that that just isn't going to happen; it would make a mess and not achieve the goal anyway. If you are able and willing to leave a helpful comment, something that hasn't been said already, it's a kindness to do so, especially for newer users who are still learning how SE works. But it's entirely up to you, and if you've already upvoted what you would have otherwise said, there's no point in saying more.