Good question. How many data points they use for the projection? One? The article title should begin with be 'Stars are pointing ....
Forget to add Tesla battery in Australia.
You misunderstand the article.
First The historic cost of diesel generators was well known from years of Operation.
The Solar + storage was a private financed project with long term PPA contract.
PPA means Power Purchase Agreement Contract.
And it means I will build this project if you (the customers) pay me as a long term contract 11 cent pr. KWh for the electricity i deliver to the customers.
In this way customer know it is secured cheaper power than diesel, and power provider is secured economics of the project.
They knew how much diesel was used earlier, So they knew the potential income for the solar + storage project.