Typically, we think of overfitting and underfitting as exclusive properties. IMO a large problem here is that the author's definition of overfitting is not inco

Typically, we think of overfitting and underfitting as exclusive properties. IMO a large problem here is that the author's definition of overfitting is not inconsistent with underfitting. (Underfit indicates a poor fit on both the training and test sets, in general.) For example, a simple model might underfit in general, but it may still fit the training set better than the test. If this happens yet both are poor fits, it is clearly underfitting and not overfitting. Yet by the article's definition, it would both be underfitting and overfitting simultaneously. So, I suspect this is not an ideal definition.