Plausibility was never an argument against Searle's Chinese Room Argument ... that's a very deep confusion, since Searle was arguing against the possibility of Strong AI, whereas advocates of Strong AI of course thought it was plausible and that's why they were working to create it. That you conflate the Chinese Room Argument with "the Chinese Room" is one aspect of the confusion.
It’s absolutely about the plausibility of mechanistic intentionality, given the implausibility of intentionality where the person in the experiment doesn’t understand chinese