First don't think of the brain as a CPU. If one has to think of it in computer terms it is system, indeed, part of the nervous system, where results of much activity is processed into a person's command and control functions. Adjusted and updated behavioral commands are initiated there which set and adjust ongoing local input and output activity. So looking at brain activity cannot tell us what a person is thinking or even what a person is doing. That always becomes apparent at the outputs of the nervous system taken together.
Of course we understand brain activity. We can predict what is going to happen next by observing it along with and through models we've developed for the rest of nervous system activity.
Your blow hole is really getting tired. All you do is point at a haystack and ask where is the pin. What's obvious is the pin is where some farmer or player on the farm dropped it for the machinery of the farm to pick up and process it along with the hay into bales which were then put in one stack or another. So the pin probably isn't even in the hay stack at which you point.
I am tired of the faithful and their lies about understanding anything about brain activity.
You cannot predict anything of significance.
If people are forced to make an either/or decision that only involves one side of the body crude predictions can be made, like which side will likely move next, with statistical analysis of the "cloud" of preparatory "activity".
That isn't an understanding of anything. It is just a worthless trick.
It isn't a prediction of anything of significance.