120 GWh is not what most would describe as "little storage". The largest hydroelectric storage facilities in existence are in the single digit or low double digit GWh. And this is for a country with a relatively small population.The current pipeline of utility battery storage projects in Australia is estimated to be 26GW ( https://www.pv-magazine-australia.com/2021/11/03/australias-... ). Storage ratios of announced projects seem to range from 2 to 4, so that would be storage in the range of 52GWh - 104GWh - not quite up to the assumed mix in the model but not too far short either.
This is also a big simplification. While dams can just totally shut off the flow of water downstream, this has big ecological impact and also impacts availability of water in population centers that draw from the river. Most dams always release a minimum amount of water to ensure the river never runs dry. Much of his simulation has dams bottling up water for long stretches of time.
Did you miss the part where he imposed minimums on the hydro generation in order to keep it within historical limits:
However, to maintain consistency with historical generation,
hydro generation was also subject to the following constraints:
* Hydro generation was kept between 200 MW and 6,000 MW
* Weekly hydro generation was kept above 168 GWh
* Annual hydro generation was targeted at between 6% and 9%
of demand, though ideally closer to 15,000 GWh, or about
7.5% of demand.
I do however agree the hydro part is a little too oversimplified (going by what is in the article). A lot of Tasmania's hydro is run-of-river; the dam-based hydro systems can be very limited by weather-based issues (both too little rain, which hasn't been concern in the previous year due to a persistent La Nina, and too much, which limits how much can be discharged when downstream pondages are too full).