The author is stating that the child dependency cannot be extracted to another library or app. If it is extracted, it is untested, because the only tests wrapping the child dependency are actually testing the child's original parent. (Which is likely to not exist in whatever other library/app to which the child component is moved.) And then, to retroactively add tests to the child component in order to facilitate moving it to another library, is painful.

Having symmetrical tests enable components to be easier moved to other libraries/apps because the test can move with the unit under test.