To be clear, I don't think the guy in the room will end up understanding whichever Chinese language this thought experiment is being conducted in, either.
You have put your finger on the fundamental problem of the argument: Searle never gave a good justification for the tacit and question-begging premise that if the human operator did not understand the language, then nothing would (there is a second tacit premise at work here: that performing the room's task required something to understand the language. LLMs arguably suggest that the task could be performed without anything resembling a human's understanding of language.)
Searle's attempt to justify this premise (the 'human or nothing' one) against the so-called 'systems reply' is to have the operator memorize the book, so that the human is the whole system. Elsewhere [1] I have explained why I don't buy this.