What if they believed everything had (some kind of) a soul, including (but not necessarily limited) to plants? The reactions would be interpreted differently. After all, magic beliefs and animistic beliefs were often associated in our history, and wizards actually have things like that crazy violent willow and the sorting hat. They also have examples of magical creatures, including for example centaurs who both have human-like sentience and are magic users (or at least capable of prophecy).

Perhaps Draco has just misspoken, or spoken without thinking. After all, racist beliefs are traditionally not very internall consistent. Maybe the actual claim is only wizard souls go to heaven, or something like that.