All plants produce some kind ‘secondary’ compound(s). These are chemicals that are always present as well as the same or others that are seasonally present. The purpose is to dissuade consumption by herbivores. It is an evolutionary response, the plant may get stuck in a dead end for producing different compounds, or some herbivores co-evolve a mechanism to deal with the compound that discourages consumption.
Most plants we find in the grocery store have been cultivated so as to reduce those secondary compounds relative to out tastes, palatability, and later on for marketability.
Tortoises would seem to have different co-adaptations depending on their range of origen. European Testudo might not do so well with toxins present in plants that are commonly eaten by Indian Star tortoises, as an example. I think there may well be compounds that they can all tolerate well, but we don’t know it. This simple matter has lead to much confusion over what is a safe feed or not.
I have read that Virginia Creeper is possibly considered not a good food item, yet I know of some keepers that feed it often and it is planted in enclosures for desert tortoises, Indian and Burmese Star tortoises, many testudo and I forget what other species. Some will run over to eat it in preference to Mulberry (say it isn’t so!).
The point is being cautious is good. No single food item should be “most” (over 50%) of the diet when feeding bought or harvested greens. But there are indeed many greens that tortoises can, do, and get quality nutrition from, that we would never eat.