Well I suppose there is some truth to that. However, even humans do not pose a threat until they show interest.
Just this weekend, my brother and I were standing right under a big old Boxelder tree talking -not being quiet or sneaky. A Pileated woodpecker comes and lands right in full view only a little ways away. We looked at it for a while and it didn't care about us. So I thought, hey let me take a picture with my camera. As soon as I tried that - GONE!
They always know!
All Cacti are succulents, but not all succulents are Cacti