That means your GM needs to unlearn what they have learned. As a GM, making too many high toughness / Wild Cards gets this result (grindy). I've seen that happen a number of times (self included) from former D&D GMs. Too bad that was your experience with the system.
I ran some zombie action in both Ravenloft and in Pinnacle's Wild Hunt. I added a bit of spice like when you hit a zombie that would just result in a Shaken fun stuff would happen -- split open their guts and intestines start to grapple you, noxious gas emits from the body, cut their legs off but they still keep coming, etc. That's the kind of stuff that creeped my players out without making the zombies "tough" or super hard to kill. Horror is not about making things hard, it about making them unsettling and scary.
While Planescape's cosmology originated with D&D, nothing is stopping one from moving that cosmology to a different system. I think your friend is right as far as that goes - the stuff that makes PS (or most other settings) interesting isn't found in the rules, it's found in the setting material. And the setting's themes may not be fully realized by a system that was designed for dungeon crawls and wilderness exploration.The way I see this is that D&D offers a lot of material that is true to Planescape. The other guy pointed out that Planechase is all about the factions and the cosmology, neither of which is specific to D&D according to him. I disagree with the latter, though: it's *the* D&D cosmology. It's a world in which anything D&D can come together in surprising combinations. Why would I disregard all that specific material simply because it requires book-keeping?