My understanding is that it’s not that simple. By your logic, humans also have several brains. for example, your spine is doing some processing on the signals from your limbs. it’s how that reflex works when the doctor hits your knee with a hammer. the signal travels to your spine, the spine recognizes some pattern, and returns a signal to jerk your leg. the signal doesn’t need to reach your head to trigger the reflex. basically, your whole nervous system is the brain, it’s just that the vast majority of your nervous system is inside your head.
it’s the same for octopi, but much more extreme. I think they have like 50% of their nervous system in their ‘brain’, and the rest is distributed across their tentacles.
feel free to correct me, I wrote this entirely based on my memory so I might be off.