I think the point is something like "don't unit test the system as a whole". Black box testing is great and most definitely should be used, but should be kept very high level and unit tests should be kept very small and granular.
Lastly, I really wish the author would give suggestions for what he considers to be a better approach to testing, beyond "integration testing sucks and is a scam".
You might be interested in contest[1], which adds a thin layer of sugar over Test::Unit.
In addition to nested contexts, it also lets you provide your test name as a string rather than a method name, to avoid wearing out your underscore key with "def test_new_instance_should_respond_to_foo".