Oxalates bind to calcium and prevent it from being absorbed and it's excremented from their chloeca. Unless that that changed in biochemistry, it's not a misconception.
We have 2 California Desert Tortoises. His diet should be more varied. I read a research paper by a leading scientist who studies Desert Tortoises and they eat like 40+ plants in the wild. There is a lot of research out there, if you can read scientific papers. If not, have you ever tried...
I have two California desert tortoises who have the run or our entire backyard. Our backyard has a steep slope that goes up to a school yard above our yard (with a chainlink fence), and they climb up that with no problem (close to 100% slope or almost 45 degree angle). One of our tortoises has...