Well, he did some conqueror's Haki at the end, but what if he did it by "accident" like Luffy did before he was actually trained in using Haki?
Also, I understood the situation like this:
There was no time to do anything but to grab Luffy. If he knew how to use Haki, he probably even saw what will happen to his arm, but that's why he gambled to save Luffy. (During Marineford, Luffy had such a premonition and didn't extend his arms or they would been cut off. But in Shank's case that would have meant Luffy dies.)
Usually in this show, someone would have just slapped the monster away in the last minute, sure. But I refuse to believe he lost his arm on purpose just to make a point.