Round trip (Australia - Beijing 20H Transit - Saudi Arabia) on Air China for A Sudanese National

Yes, this is correct. Your itinerary, under the condition that it is presented accurately, is definitely and undeniably eligible for 24 hour visa free transit and you do not need any kind of visa.

Having in mind that your layover in Beijing is a very long one, you could also think about exiting the airport and to enjoy a night rest in the hotel. If this is the case you will approach 24/144 hour visa free transit counter immediately upon your landing. At the counter you will apply for a temporary entry permit. This application is free of any charge and here you will present your flight tickets and your hotel booking accommodation. You have to be aware that granting a permit depends solely on the discretionary decision of the airport official on the day, but having in mind your very long layover, I would say that your chances to be granted a permit are really very good.

When your permit is granted you will proceed to the immigration and then to Beijing to enjoy some sightseeing and also to have a good night rest in the hotel of your choice. Have a pleasant journey.