I was the one (or at least one of the people) who flagged that answer as link-only. A moderator saw the flag and must have agreed. My reasoning was, if the link dies, the answer becomes completely useless. Links are supposed to supplement the text in an answer, not replace it. As for allowing you to update the answer, you can. You are able, and encouraged, to edit the answer and undelete it yourself. Unlike questions where a closed question requires a community consensus or moderator action to re-open, a deleted answer can be re-opened by the owner of the answer at will.* Think of it not as being deleted so much as being sent back to you for improvement before you release it back to the world. Just make sure your edits are substantial enough that it will not be deleted again.
When you look through old posts (both questions and answers) around this site and sites like this, there's a trend of links being vital to understand an answer, yet said links return 404 or don't even point to an extant website anymore. To avoid such a fate, you should at least summarize what is past the link. This can include anything from a quote from the link to an entire paragraph analyzing what is being discussed on the other side of the hypertext. Use your own judgement.
Take an example of an answer which I posted. The question was why a certain decision was made for Ubuntu, and the answer was present on their own website. Instead of linking only to the website, I wrote a quick summary of the reasoning, and then quoted the relevant sections. If the blog post which contained the canonical answer (pardon the pun) was ever to vanish, the answer would still be just as relevant. If I had only mentioned that the reasoning was explained on the blog and then linked to the blog, my answer would have likely been deleted, too.
* This is assuming the answer was deleted in the community review queue. You cannot undelete an answer that was deleted directly by a moderator on your own. You can, however, edit it and flag it for undeletion.