These close-to-even ratios make for a more powerful test of classification. I would assume that these test samples have biopsy data means that some dermatologist thought that they might be malignant (unnecessary medical operations are unethical). This might lead to some bias towards samples that are difficult for humans to diagnose.
Separating these into binary classifications of specific tumor types makes it easier to classify than out of every possible tumor type (as a dermatologist does).
Still the claims this paper makes are very promising. A lot of the training data was classified by dermatologists, not biopsy. Using more biopsy data could lead to even better classification, as well as improvements to the model.