In the photo on the Food52 feature for this recipe, it looks a bit like you used bok choy instead of cabbage. Would that work for this recipe?

All excellent answers - the actual greens are tender napa cabbage which is a variation of mustard greens only available in Korean markets. That said, I've tried bok choy but a long fermentation gives off some bitter flavors that I did not care for. I agree that trying different types of greens and experimenting will give you best results. ALL DELICIOUS is right!

Bok choy is close to napa cabbage in lineage, so hidden-closet cousins can be exposed! As aargersi says any sort of greens will work.
Mustard greens are thinner --done in a day or two, thicker napa cabbage bottoms- 10 days or more to cure. ALL DELICIOUS

I think it would! I am about to make a batch using regular cabbage and mustard greens from my CSA - I think any sort of greens would work, you will just get different flavors and textures. Try it! Try a bunch of stuff!