We only understand about 10% of how the brain actually functions yes, which is what it does and how it does it. But the underlying structure of what allows it to function, sort of like the blueprint / the design of the cell structures / neurons etc, we understand quite a bit more. A VERY important distinction to make.
Think of it like a computer in this hypothetical scenario, imagine knowing the layout of the computer, you have your CPU, RAM, GPU, motherboard etc and we understand that, we understand the physical architecture inside the microprocessors and chips and hard drives, the layouts of the transistors etc, then we also understand that the computer deals in 1's and 0's. All of that is akin to what we understand about the brain. Non of the above mentioned things would allow us to understand what the computer does and how it does it, but the underlying blueprint that in turn provides the platform, to allow it to do what it does. Even though we do not understand much about what it does and how it does it, with this information, we can build a virtual brain (neural network) if you will, then experiment.
What we do not understand about the brain, which we do about computers, is once we have the design as mentioned above, is then how the data is manipulated inside the computer to make the thing work. Which can be compared to the the functioning of the brain, that is what we only know 10% of.
Then through trial and error, the networks themselves, over time, work out what to do, how to do it and how to function. They have started doing this VERY successfully now. We are after all, basing our designs of neural networks on the physical design of the brain, something we can visually see, through a microscope. So they are fundamentally based on a design, as designed by nature, so it is no surprise they can and have started to fill in the blanks so to speak.
Because we know much more than 10% of the physical layout and design of the cell / neuron structures, by looking at them through microscope's and what not, it has allowed us to build AI neural networks based on the said blueprint above. So no, I disagree with the "how the brain works" is fundamentally flawed, that argument itself is fundamentally flawed for the above mentioned reasons.
Ironically, AI, which is designed on the brain structure, is now unlocking more secrets about how the real brain functions too. Which is only getting better over time. It is coming full circle. Again, going back to the knowledge age we are now in.