There were two parts in the answer you flagged: one part that said in essence “I agree with the other answers that recommend Observer”, and another part that recommended Rx.
If the post had only consisted of the first part, then this would have been an invalid answer, that should have been an upvote. A simple “not an answer” flag would have sufficed; I sixth the opinion that your flag is unclear and I wouldn't have understood what you meant.
But the post had another part that none of the existing answers mentioned. For that reason, deleting the answer would not have been appropriate. If Rx is not a good answer to the question, downvote.
Furthermore, it is perfectly legitimate to lend your voice to advice already provided in another answer, when it's part of a post with added value. This is best done with a link to the answer that provides the more complete advice.
If there's already an answer that says “do X and here's how”, and you want to say “do X and Y”, then it's completely normal to
- either write “do X (see [bob's answer](link) for details). Then, do Y, here's how…”
- or completely explain how to do both X and Y, if you don't think the existing answer makes a good enough job, or you think it's better to have all the information in one place.
P.S. When you discuss someone's post on Meta, please link to the meta discussion in a comment so that the person knows it's happening.