Thanks that helps. Though I still have concerns...

the chinese room experiment is not asking whether this is possible (i don't think) - it's asking whether, if this scenario were taking place, the book + the person somehow contain the semantics of chinese speech. how could they not?

If we were to only ask for the correct output to the input, then the AI passes the test, transforming inputs into believable outputs. But if we care about the function that transforms input to output, and I think we do, then how X becomes Y is important. I think to "understand" includes reference the function, which in the case of language requires a human interior or something that's the same in its workings.

If we asked to demonstrate that we understand a mathematical concept, we are asked to show our thinking, our thought process, not just the correct answer. By metaphor, if you're given a few logs (input) to solve by hand (function) in order to show (output) that you understand the concept, you don't then go to a gigantic log table (mere output) because that doesn't demonstrate (function) that you understand. Eventually, you'll learn them well enough that externalizing your internal thoughts isn't a necessary demonstration, but if in 6 months you can't remember how to show your work then you've clearly lost some measure of understanding. So if by understanding, we only mean converting inputs somehow into believable outputs, then a gigantic log table would be fine. However, we do think how we get somewhere matters, but for language we unfortunately can't show all of the important bits completely, fMRI aside. Thinking is an essential aspect to doing language. To know language is to have thoughts. So when you ask...

why do you think it's important?

It seems obviously important to me... it would be absurd to say that I "know" a language without being able to think in it.

If instead of language we were talking about pain, then the problem becomes much more pronounced, since many people would say that if something experiences pain then we ought to treat it differently than we would otherwise, and of course pain is only relevant if it is actually experienced not fibbed.