If consumed in small amounts, the oxalates (sp) in cabbage and other dark green oxalates is almost harmless. Adding it to their meals once in a while for variety sake will do no harm at all.
Kale too, like other dark greens conatins oxalates and can be fed as part of their diet in small portions as with everything.
Can Hermanns Tortoises eat Cabbage?
Yeah. However, their primary diet should be plants and weeds, as a treat they can get meals like cucumber or tomato once every so often, as I’m sure everyone is aware.
ALSO SEE: Can Hermanns Tortoises eat Bell Peppers?
Personally, I have fed mine carrot, cabbage, kale, and the likes for almost 12 years now. Remember, variety and litltle amounts is key here.
So, feed different plants, weeds, and every so often something like cabbage or kale to change it about a bit and add some additional nutients that may be absent from their main diet.