Hi @scroobius-pip,
Thanks for your question.
Yes, it appears that setDropOp takes a number. I was looking at relevant code in dgraph-js/api_pb.d.ts at master · dgraph-io/dgraph-js · GitHub .
I tried to reproduce this and was able to delete data using op.setDropOp(dgraph.Operation.DropOp.DATA);
I have also reported this back to be included in documentation or example files.