I understand that you haven't committed between inserting and deleting the rows. If so, this is why it doesn't work - flashback queries only see committed changes to the database, never uncommitted changes, even if they were made (and then undone) in the same session.
There can be a flashback data archive (sometimes called Total Recall), but even if there isn't, flashback can always use undo. Besides, when the data needed for the flashback is no longer in the undo, an error occurs. Otherwise you couldn't distinguish a table that was empty two minutes ago from situation where there are no data for the table at two minutes ago.
I just did a quick test that shows that a missing commit is a problem: