The document you are referring to is outdated. It contains the old DCI tournament policy. 512 players is 9 rounds.
7-1-1 is guaranteed Top8 with 512 players.
Sorry Toad, but there are no guarantees. That's why tiebreakers exist in the first place.
As CH1N5 and Benga alluded to, there is definitely no guarantee X-1-1 will make it, and there usually is a circumstance where people won't make it. This is why they have added an additional round (or TWO) at most Pro Tours now. So if they had 257-512 players, rather than playing the obligatory 9 rounds, they would now add on another and play 10 (usually). This helps to define who are truly the best and most deserving players, and that extra round really does make a difference, especially if you're in the Top 32 going into the last round. Per the Wizards website,
Everyone agrees that more rounds make for a better competition and both days still ended at a reasonable hour. This change has now been adopted as a permanent change and future constructed PTs will have 8 rounds on each of Day 1 and Day 2. Even though this would be a one day event, I don't think 10 rounds would be too much if you already have planned 9.