She kept them in there for about 3 years and then upgraded them to a 40 breeder. She posted this video right before she moved them to show how large they got in the 29 gal.
I also find a lot of goldfish tanks pretty bare. But you kinda have to in a tank like this. The substrate and decor take up a few gallons of water, knocking something like a 29 gal tank down to a 25 gallon tank. But it's also common with people who want a low-maintenance tank; poop is a lot easier to suck up when it gathers behind the only two ornaments in the tank. I know I will never have goldfish in a bare-bottom tank, tho. They LOVE playing in the sand; it's literally 40% of what I see them do on a daily basis. The other 50% is beg for food and that last 10% is graze the algae.