Where did you see this? From all accounts, he’s a stabilizing presence on the blue line. I haven’t read anywhere that he is “really bad” defensively.
This is from John Hoven report from last year:
Known as primarily a puck-mover, Spence is starting to find a greater appreciation for the defensive side of things.
Like most prospects his age, he’ll need to find another step in his skating, add some speed, and needs to build strength (which will also enhance his skating).