> It’s also why ruthless testing and 100% test coverage have become so important in mainstream languages. But even with 100% test coverage you can’t be sure that your code will work correctly if a function unexpectedly returns nil unless you’re also mocking things out or using fuzz testing.

You can write tests with 100% coverage that don't use a single assertion. Test coverage is a completely useless metric, but it is an easy one to measure and understand, which is why it is so popular in pseudo-QA and the management tier.