Here's where I agree with his statement:
On the backend, I agree that all of the domain-layer code should be tested. This is a hard requirement for me. It's also code that has 0 dependencies so it should be easy to test. TDD pairs really nicely with DDD.
Unit tests give you the needed confidence in order to do refactoring. You can't refactor code without tests. If you do, there's a risk involved. Therefore, in order to safely improve the design of existing code, it needs to have tests. This "detrimental rumor backed by zero evidence" is one of the fundamental takeaways from Martin Fowler's book on "Refactoring" in addition to Uncle Bob's chapter in The Clean Coder on Unit Testing.
Here's where I disagree with his statement on 100% test coverage.
I used to spend a lot of time writing brittle tests by testing front-end UI code. I used tools like Selenium and Cypress. Because the front-end is the most susceptible part of a system to change, I found myself spending an equivalent amount of time maintaining these tests in addition to adding new code. This is a hard place to find a balance. In Angular, I merely ensure that I write tests for services. In React, I spend a lot less time writing Enzyme tests on rendering and a lot more time testing the redux operators.
Uncle Bob clears this up in his book by saying USUALLY, it's not necessary to write UI tests. I'd say by UI tests, he means "rendering tests". His solution is to ensure that you have a way to run acceptance tests that work through the API, as it should be a lot less susceptible to constant regressions. This way, you're essentially testing the features that the API is executing. If you're using DTOs, the inputs and outputs of your system should remain relatively the same anyways, and you should spend less time changing old tests, and more time adding new ones.