Maybe for some Configure to order / Build to order (CTO / BTO ) options . Fall (which is bounded by Dec 20th ) is only for shipping something; not necessarily everything to everywhere.
Apple is probably building 1-2 "standard configurations". Those will highly likely ship by Fall because Apple will simply just push a few out the door to hit the deadline. That doesn't mean they will be in readily available supply or that all optional configurations will be ready by that date.
If the initial demand bubble super slams the available supply Apple will probably cut some countries off the availability list too.
But there is zero reason for Apple to wait past the 20th and slide to the 31st. There would be something made in the factory even if they started after USA Thanksgiving weekend by the 20th. (which is unlikely. Production is probably already running if they are lining up some dog and pony show for the press with the Trump jumping on the bandwagon. )
The problem that Apple likely has at the moment is the the initial demand probably is going to far outstrip the supply. So they need lead time to build up some inventories so the shortages are ridiculously long and burdensome. However, they probably aren't shooting for zero shortages either as they can spin some mismatch in supply-demand as "surprisingly strong" demand. ( a bit like encouraging lines for new iPhones without the lines getting too ridiculous. )
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Meteorology has their own Winter.
https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons
Apple is talking about Calendar time. They don't have to wait until Fall cold weather to harvest the Mac Pro crop. The weather has nothing to do with making a Mac Pro.
Time, ,availability of parts, and logistics does. That is calendar time ( which is derived from astronomy. )
The only other time scale Apple may talk about is a fiscal scale ( their FY starts in September. ). ... which again has little to do with weather.