Shouldn't we mention something like:

"OOB estimates is a pessimistic estimator of the true test loss, but remains a fairly good approximation for a small number of trees. As the figure indeed shows, it tracks the loss on the test for the first hundred iterations but then diverges in a pessimistic way."

I think we should also say while it is a pessimitic estimator it, at least, comes for free without holding out a validation set.