That is not really a cut-off. You can continue to ship inventory well after that time, but due to receiving times, congestion, and other considerations, anything received at the warehouses after that date might not be received in time for orders that would be delivered before Christmas.
However, that is never an assurance. Some warehouses go overwhelmed and end up diverting shipments to other warehouses or to depots to wait for later delivery appointments.
Suffice it to mean that anything that arrives after the end of October will face some delays along the way.
What is not mentioned is that LTL shipments will have even greater impacts. SPD often will be received faster than LTL and have less issues, but that will never be guaranteed, either.
I will always continue to restock where necessary continuously knowing that some items will not be sold before Christmas, but will continue to be available for sales after Christmas. January is when customers spend gift cards or money/checks received as their gifts, so ramping up for Christmas day is not the point.
Additionally, you will want to manage your inventory to be at the correct level for after the Christmas season. That might include some after Christmas clearance or additional restock.
The “in time for Christmas” is really “ready for purchase” before the 2nd week of December so that orders (especially Standard Shipping) can arrive before Christmas. After that time, only premium (additional cost) shipments will have much of a chance and last minute shoppers will need to be more to get their orders delivered before Christmas. All you can do is get your stuff in as early as possible to be ahead of the rush. The penalty for that is higher storage costs while they are at the warehouse.