The key point is that the referee considered that Alexander turning and reaching the ball into the endzone was a "football move" and established possession.
Whether you agree that he actually had possession, is another issue and it's a judgement call. But considering that the referee made that judgement call, he followed the letter of the law in calling it a touchdown. It would have been no different than a running back reaching the ball into the end zone and having it pop out when he landed.
Had he had caught it in the end zone, i think it's harder to say whether or not it would have been ruled a touchdown because what determines possession in the endzone, I don't think anyone really knows. It's still a judgement call and for whatever reason the NFL is stricter about catches in the end zone than anywhere else on the field (why isn't it consistent no matter where you catch it?)
Think about the Calvin Johnson incident a couple of years back. (I think virtually everyone thinks that was a TD.)