I know this is a discussions for tanks, but let me play Devil's Advocate here. What if the Tank is new, and they say nothing? Is it on them?
I know what was already said about assuming, but I went through an AK HM run a little while ago. Tank was geared enough, though a bit sluggish. First boss, he tanked near statues and didn't hide when he got targeted. Naturally, we wiped.
Because of the aforementioned tank stigma, I assumed he was a troll and left. The possibility that he was new didn't occur to me until after.
In short, I agree with the sentiment that people need to speak up if they're new.