You are expecting Microsoft to test a nearly infinite number of variables to find every flaw. This is not a reasonable position.
Your statement would also make it unreasonable for Google to be able to find this stuff, but we have existence proof that that's not the case.
His position is absurd. Microsoft could have found it. The thing stopping them is the incredible number of tests you would need to perform to find them all.
The OP asked why Microsoft didn't find this first. The answer is too many things to test. The entire premise of the argument is insane. If microsoft had a viable way to test ALL of the their products for ANY problem, don't you think they would be doing that?