If the "unit under test" is low level then thats coupling low level implementation details to the test.

If you're vague about what constitutes a "unit" that means youre probably not thinking about this problem.

Often, even outside of software, unit testing means testing a component's (unit‘s) external behavior.

If you don't accept that concept I can see how TDD and testing in general would be challenging.

I general it is most productive when building competency with a new subject to accept the author's definitions, then adjust once you have experience.

IMHO, the sizing of components is context, language, and team dependent. But it really doesn't matter TDD is just as much about helping with other problems like action bias, and is only one part of a comprehensive testing strategy.

While how you choose to define a 'unit' will impact outcomes, TDD it self isn't dependent on a firm definition.