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.
"While WARN requires only 60 days’ advance notice, Amazon is providing at least 90 days’ notice to all affected employees before their separations are scheduled to occur. Affected employees who accept internal transfer opportunities at Amazon prior to their separation date will not be separated as a result of this action."
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.
The date of the notice is October 28th. The separation dates given in the letter are 90 days out from the date of the notice (for some technical reasons, some employees had a separation dates a bit further out.)