The question means all of our Bitcoin at Coinbase. He's not making a conspiracy theory out of it - he's asking a legitimate question about whether or not Coinbase has all deposits accounted for or not.
That's a rationalization around the original blaming statement you made. You asked how we should know if they are secure. He said how do you know they haven't been hacked already? Both legitimate, up to the point you said he was making conspiracy theories.
I assumed he was talking like they had just spent the bitcoin (Mt Gox stylee). Even if they were hacked, I would expect Coinbase to disclose that information--to presume that they've been hacked, lost bitcoin, and are hiding that from everyone is a pretty wild theory in my book.
