I find it difficult to believe that flagging a post automatically gets it deleted ...
It doesn't. A flagged post is reviewed by a moderator and deleted if it's not consistent with the rules. Posts aren't deleted arbitrarily, although that's not to say that a post might not be mistakenly deleted. If you ever have a question about the process, you can reach the moderators using the Contact Us link at the bottom of each page. They're good people.