It’s safe indeed to turn off the depth test if you don’t have a depth buffer.
[[CCDirector sharedDirector] setDepthTest:NO];
That’s something that needs to change in the code as well. Turn the depth test on only if the EAGLView is initialized with a depth buffer. Not that the current situation will affect performance, the depth test can’t do a thing without a depth buffer.
I would only turn on the depth buffer if you need 3D effects. Otherwise it’s a waste of memory.