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While you develop a feature, you do a lot of tests including white-box tests with the debugger, that would be annoying to automate and maybe wouldn't even survive the next commit. You also do "tests" by manually executing the code in your head modelling all the execution states. Automated tests often only test single cases, while rationing across all states, is hard to automate. They likely also had a specification next to their editor, referencing all the nitpicks in there and proving in their head that these are indeed addressed.

These kind of "tests" often are enforced as the codebase evolved, by having old guys e.g. named Torvalds that yell at new guys. They are hard to formalize short of writing a proof.