The Fed announcement may or may not have been known, but the fact that there would be an announcement at a given time was known. The market always jumps one way or another at the last moment before an announcement, because it's the last chance to place your bets in what is essentially gambling. There was no information propagation delay because there was no information. Sometimes the blind consensus guesses right, sometimes wrong.
If you actually had leaked information there's no reason you'd need to wait until the very last moment, that's when people who know nothing place their bets and their actions would be more likely to mess up your scam than cover up for it.
Indeed - it's worth noting that on the non-farm payroll figure (which is closely watched by markets) this month, most markets jumped in the wrong direction a split second early, before violently reversing.
If you actually had leaked information there's no reason you'd need to wait until the very last moment, that's when people who know nothing place their bets and their actions would be more likely to mess up your scam than cover up for it.