When I finally decided to really decouple and mock dependencies and create "pure" unit tests was the major "Ah ha!" moment of TDD. I had been trying it out here and there but never really saw the benefit because I wasn't really creating nice testable units.

It is hard to say for sure, because the author doesn't give any specifics but wording like:

"You therefore are more reluctant to make large-scale changes that will lead to the failure of lots of tests. Psychologically, you become conservative to avoid breaking lots of tests."

make me think the author wasn't actually making unit tests, instead they were likely end-to-end tests, or partial ETE tests that were running inside a unit test framework. I have had many disagreements with other developers that just because your test runs inside a unit testing framework doesn't actually make your test a "unit" test.