I think a sigma can be both introverted and extroverted, but it works better with introversion than alpha due to ignoring the social hierarchy.
I'm a sigma according to the definition. Extroversion just means I enjoy talking to people, but it's for my own entertainment rather than playing social games for status and validation like an alpha does. I have no need to dominate others, but I despise being dominated by anyone other than myself and will not allow it.
The sigma male is often underestimated, but not even an alpha could make me or any other sigma back down from exercising our autonomy. I've been underestimated many times because I'm confused with a beta, but they figure out soon enough I have more of a spine than most people if they try to push me. I respect other sigmas more than I respect alphas, the latter often look like clowns to me.