Apple avoids shipping new hardware in November/December because they want their holiday lineup to be set by then. There is zero chance of January, don't even know why that's a poll option.
avoids November December? Gerernally but not particularly rigid rule.
iMac "thinned edges" transition ( 201? ). the 27 models pratically slid out into Janurary when Apple had manufacturing problems.
Mac Pro 2013 December.
iMac Pro 2017 December.
MacBook Pro "16 2019 (late) November.
Mac Pro 2019 December.
The 2013 , 2017 , 2019 were all vague promises by Apple in June of that year that about something before the "end of the year" or "before Winter " (meaning the Winter solstace December 19th - 21st range ).
Apple's "June 2020" promise of before the end of the year rings very much like their Mac Pro and iMac Pro "sneak previews" ( only "preview" is just highly limited to a non-product DTK in terms of physical shape. Not a preview of actual product. )
In fact, Apple deliberated bailed out of the now-defunct Macworld Expo because they did not want to be forced to come up with new announcements in early January. That worked again the retail buying cycle and was problematic with employees taking time off (Hannukah, Christmas, New Year, etc.).
That's not really what Apple said. They said they were bailing out so that could release products more so when they were ready instead of some arbitrary calendar timeline. Of course after the first couple of interations on the iPhone lapsed on this to exactly go back to some rigid fixed in stone "arbitrary" deadline date (only of their own choosing.) .
It would be quite easy if planned in advance to finish before Holidays ( Hannuak , Christmas , New Year). [ Probably would leak in more detail is wrapped up a coupled weeks in advance but also ] Chinese New Year is a big manufacturing break in January also.
The more mainstream Mac releases have moved around over time.
The most realistic Apple Silicon Mac release window is the latter half of October.
Given the pandemic the more realistic window is probaby mid-late November earliest. Apple was probably originally aiming in their initial plans for October. if the iPhones are sliding into October, there is no real creditable expectation that the Macs are not also sliding farther back. WWDC was late. The macOS these system will run on will be later than normal also.
Let's also remember that Apple did not refer to the new devices as ARM Macs. They specifically and repeatedly called them Apple Silicon. During the WWDC keynote, Apple made a great effort in not highlighting the Arm architecture. They were also very consciously vague about the GPU architecture during the keynote instead rather blandly using the "Apple Silicon" phrase to describe the entirety of the hardware.
That is just branding. The same reason Macs don't have an "intel Inside" stickers on them. ( of "AMD inside" stickers for GPU). Apple is trying to extent pushing the Apple brand. It is also a precursor to not being able to boot anything but Apple macOS.