It wouldn't because the way that is worded suggests that the reason for dismissal is the credibility of the person and not if the implausibility of the idea.

What would follow from his first statement: "Your idea sounds really implausible, and so I'm going to dismiss it on that basis. And since you're not a domain expert I'm not going to put special effort into suspending judgement, I'm just going to go with my initial implausibility determination."

What would follow from his second statement: "I'm going to dismiss your idea because you're not a domain expert. The idea's plausibility didn't really factor into my decision."

I would agree though that his use of very simple language has left some ambiguity here.