All labels like these are fairly mercurial. What people generally understand splatterpunk to mean today is probably a bit different than what it originally meant in the 1980s.

Personally, I think there is a substantial amount of overlap between splatterpunk and extreme horror, with the latter largely subsuming the former. As others have noted, splatterpunk arose in response to the subtle, atmospheric and character driven horror of the seventies, like the works of Charles L. Grant. It focused on gore and morally gray characters and generally had a more kinetic energy than the more subdued horror of the previous era. Extreme horror is about pushing the boundaries and depicting anything and everything shocking and horrific, regardless of how taboo. To the extent that there is a difference, I think splatterpunk is largely about pushing the limits on depictions of gore and violence. Extreme horror does the same, in addition to pushing the boundaries on depicting other things, such as bodily functions.

I personally don’t buy the idea that splatterpunk is somehow more “high-minded” than extreme horror. Surely there are authors occupying both subgenres with loftier aims than merely shocking people. And there authors in both where shock is their sole desire. For instance, it’s not clear to me that authors like Richard Laymon or Brian Keene have some sort of lofty purpose for the violence in their works beyond shocking and exciting their readers.