Do mean safe to eat, or safe to have in their enclosure? I know people in AZ who have lots of this in their sulcata pens. Doesn't seem to be a problem for them.
It is safe to plant it in an enclosure, because it is a native plant to desert tortoise areas, however, I wouldn't pick leaves and mix them up with the tortoise's food. If the bush is growing in the pen, the tortoise has the choice to eat it or not. If you pick leaves and mix them up into something he regularly eats, her might eat it and it might not be good for him.
It is poisonous to livestock if they eat large quantities of it, however, this comes from a list of plants not to feed to mammals.
I have never seen the CDTs here try to munch them..however they do love to burrow into the root system ( in the wild this is an imperative plant for them to burrow into the roots...)..