Your edit doesn't really improve the post very much. You go from having a post with a broken link, to a post with a message that it used to have a broken link. Neither of these are useful. Instead, I would suggest you remove the entire last line of the post. External links should not be essential to the post. You should usually feel comfortable removing them if they're dead and it isn't possible to find an alternative.
The rejection message about "attempting to reply to the post" was because pointing out a broken link would be appropriate for a comment, but not an edit. Once you have enough reputation, you may comment about a dead link to encourage the post's owner to fix it or remove it themselves. I would do this if I can't find an alternative myself, but the original poster still participates and may be able to.
Edit summaries doesn't show up as comments to users reading the post, but they are displayed to the users who review your edit. Your edit summary was fine, please continue to include one.