1. They're animals, and incest isn't uncommon (especially for tortoises)
2. With 800 offspring, there will be more than enough genetic variety before defects have a chance to happen.
Also, reptiles have a higher frequency of genetic variation in offspring. This is why this wouldn't work in a mammal. You can do some seriously fucked up shit with reptiles and still get viable offspring... just look at the exotic snake market.
“You know all it did was have sex multiple times with other tortoises, right?“
“Shut up, Frank!! It‘s a hero!“
2. With 800 offspring, there will be more than enough genetic variety before defects have a chance to happen.