The blood brain barrier is a deny by default firewall. If there is no transporter configured for a particular molecule, it doesn't get through. There is no transporter for serotonin
See: vagal neurons and vagus nerve. Serotonin from gut directly impacts those. Your brain still directly interprets that signal from the vagus nerve and uses that to up and down regulate mood and arousal. Impact is still significant
To extend the metaphor, the brain may have a robust firewall, but it also transacts with millions of clients over a separate (electric rather than chemical) network.
