The fact that we both use Salesforce does not matter. It’s internal and doesn’t mean anything outside the company. Both the brain and gut re-used the molecule for their own internal signaling. Evolutionarily it was cheaper to use an existing molecule.
To the brain, the invoice is just “I’m full” or “I’m hangry.” It doesn’t care how much serotonin the gut had to produce internally to issue that “invoice.” The brain will produce its own serotonin from the signal of satiety but it won’t give you any more than you can from just feeling full.