As per u/Papaofmonsters, Schulz saves the first batch of slaves, and Django doesn't actually do anything extra to save the slaves near the end.

Tarantino opens the cage and throws some dynamite in with the slaves. He does then reach for the cage door before it cuts to him picking up the next batch of dynamite. After Django then kills everyone, he approaches the cage and the door is still open. He needed to kill the slavers to get away, so he doesn't get abolitionist points for that. And he didn't open the door for them, because Tarantino's character didn't actually lock it before he died.