And will very quickly humble even the most diligent of testers :) For example, you're testing that this value is in a range - but are you testing both endpoints and whether they're inclusive or exclusive? In faster languages I enjoy quickcheck style testing which also reveals these things pretty quickly.

Do not confuse end-to-end functional tests that this article is describing for smaller “unit” tests that you typically might express in quick check/mutation testing (at least as I understand the space).