I'll admit, it looked like he was off and his body language was awful. But every single coach has access to every warmup pitch he throws, and I find it hard to believe that he could fool all of them before stepping on that mound.
My guess (uneducated and uninformed as it may be) is that he tweaked something on one of his first pitches, and thought he could work his way through it. Hyde and Ebel came out, and in the end it is THEIR decision to keep him on the mound or not. Until we have proof that Kimbrel was nefariously hiding an injury, it's all speculation.