No, it is not true that we could bite through our thumb as easily as a carrot. Our brain doesn't stop us from doing so; rather, our brain prevents us from attempting such actions in the first place because it recognizes the potential harm and danger involved.
The human thumb is composed of bones, ligaments, tendons, blood vessels, nerves, and soft tissue. While the human bite can be relatively strong, it is not strong enough to bite through our own thumb as easily as a carrot.
Biting through a carrot is possible because it is a relatively soft and fibrous vegetable, and our teeth are designed to break down plant material. However, our teeth are not designed to break through bone and other hard tissues.
Attempting to bite through our own thumb would cause significant pain, damage to the thumb, and potential infection. Our brain's protective mechanisms, such as pain perception and reflexes, would kick in to prevent us from causing such harm to ourselves.
In general, our brains are wired to protect us from self-inflicted harm, and instinctively, we avoid actions that could cause severe injury.