I looked at this a bit more in detail.
What happens it the following, starting in the datetime fromtimestamp fragment :

converter = _time.gmtime if utc else _time.localtime
y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)

That will call the system gmtime_r(), which indeed returns Sat Jan  1 00:00:00 10000

I have not looked at the cpp implementation for that method and I am not sure if this is something that can be improved.