It's a completely new account that was created specifically to write a negative comment in a post. Maybe that's enough for the flagging?

Think about it this way. If a user gets enough upvotes they get a link to "vouch" for a dead post and possibly undelete it. Would you vouch for that post? Would you say "This kind of post is what we need on HN"? I think it adds little value, even if there's "nothing wrong with it".