You're making a common mistake. WARN is for the _previous_ layoff. The way they execute these, they bump out your effective last day such that there is no WARN notice till after it's already announced.
These people were told they were being laid off in October but remain on payroll. What Amazon is "bracing" for is another one of these announcements, much larger, announcing people who will actually separate from the company 90 days later. They will find out on or around the same day the WARN notice is posted.