We know the brain is doing information processing. Our senses are just streams of information mapped to our nerve signals. Our brain is constructing our environment, our memories, and our inner feelings. Damaging certain areas of the brain interrupts the information processing and predictably impairs specific functions associated with that area.

While we don't know the specific set of functions that form the basis of intelligence, we know there is information processing going on and therefore intelligence is computable. We don't know exactly the abstraction level at which neurons are forming this theoretical Turing Machine, however unless there are any reasons to think the brain is doing more than just information processing, we can conclude that machines will also be intelligent.

Then there is the definition of intelligence which I think lacks precision. There are many definitions but to continue with the provided definition, for example, "thought" and "reason" are not precise and is presuming a biological entity as the only ones having the capacity to be intelligent. Either we make that an explicit condition, meaning the definition of intelligence is something that biological brains do, or we define it such that it can apply to anything even non-living entities. If we do the latter, I think large GPT models are approaching a form of intelligence. The Turing Test is soon to be surpassed with large GPT models, where most people will not be able to tell the difference, and at that point we will have to admit that it's at least a certain type of intelligence, although perhaps not human.