There is a gap between the T20I series and the test series, and I guess they have kept it so, to accommodate the T20 World Cup. So the BCCI is indirectly hinting that the T20 World Cup would go ahead as planned in November 2020 and the IPL 2020 would be postponed (or rather cancelled). It is not practical for the players (both Indian and Australian) to travel from Australia to India in mid-October (for the IPL) and then return again for the test series.
So this fixture list answers a lot of our questions.
All this said, I am disappointed that the Optus Stadium in Peth has been overlooked for hosting test matches. I would say that it is the best stadium in all of Australia where cricket matches can be hosted. Really disappointed.