Vouching for a good comment that happens to be dead is fine—that's what vouching is for. Posting commentary about it is not fine. There are plenty of non-bullshit reasons for comments to be dead, including that an account is banned for having abused the site.
We created vouching because some banned accounts sometimes post ok comments; indeed some banned accounts only do that when they're banned, and immediately start posting abusively the moment they're unbanned. Life is complicated.
Because you're correct: if CF's info sec team is "very very good at their jobs", how did this incident happen?