I'm not so sure. I mean obviously this is very complicated and we still know very little of the brain.
Just one example (from what I understand) you can't really separate intelligence from emotions. There are researchers in emotions that explain our whole system of affect/feeling/emotion/mood/etc. as a very effective abstraction of all the inputs from our whole body (both direct inputs, cognitive processes and stored data). So without a working system of emotions I don't think you will have a working brain at all.
An older but still popular view of the human mind separate the brain and body as two parts. And we also like to separate emotions and intelligence. From my amateur understanding that does not correspond to modern neuroscience, it is all a very integrated system. We usually like clear and separate categories instead of fuzzy complicated integrated systems.
Those are two very different goals and I don't think one is necessary to achieve the other.
At least, there are people who have lost most of their gut due to disease or trauma, and their human intelligence is still working unharmed. You may say it is needed for development of the brain, but it does not seem to be needed for function once at least partially developed.
We just need to maintain our awareness of that difference.
The good news is, it makes replacing humans - trait for trait - far more more complicated.
I wouldn't take much comfort in that. You can probably reach something that resembles a human by just providing an alternative set of signals. We already do this with the reward mechanism during training and that is proving very effective.
To give a parallel, your high blood pressure may not be due to your heart, as it may be responding to a system of factors. Vasodilation and vasoconstriction of blood vessels have a direct impact. Your kidney health, an organ outside of your circulatory system can trigger high blood pressure. O2 and CO2 levels in the blood can trigger a response.
Looking at the brain in a vacuum will only get us so far.
What is the basis of this claim?
It still is though, you will reach human intelligence by just modeling the brain. The problem is that you probably won't reach an emotionally stable human since you'll be missing all the emotional/hormonal signaling that comes from the rest of the body, but we can emulate that without having to model the full complexity of the gut.