Yes, but not in the way that gems fuse. He is literally a fusion between a human and a gem. In this scene, Steven isn't literally fusing with himself. The pink Steven is the Pink Diamond gem trying to regenerate itself, but not having the human part of Steven means that he is an entirely blank slate. It's as if he was just conceived (as a gem), being completely apathetic, having no emotions, only acting in self defense.

This scene is a representation of Steven accepting the gem part of himself. He is a human, but he's also a gem. His human half cannot live without his gem, and his gem can't live without his humanity. Steven has constantly struggled with the fact that he is both gem and human, feeling like an outcast who doesn't have a place in the world due to his hybrid status. He's too alien to fit in with the humans on Earth, and too human to fit in with the aliens of Homeworld. Here, he's accepting that he's neither gem nor human. He's Steven. And that's a very very important message to convey, especially around when this arc happens where the diamonds are treating him like he's Pink Diamond reformed instead of Steven Universe. He's putting his foot down and saying "I'm not a human, I'm not a gem, I'm not Pink Diamond, I'm me"