I think you're reading something that isn't being argued in the post, though. The author is not arguing that you will have no bugs in your game if you use ECS a

I think you're reading something that isn't being argued in the post, though. The author is not arguing that you will have no bugs in your game if you use ECS and add tests, it's that testing in this way can make it easier for development because you reduce the possibility of introducing a bug inadvertently in one of your systems and then have to rely on QA to find it later (components too I guess, but I would guess component "tests" can be covered by types since they're just data and the systems are the ones that interact with the data). The kinds of bugs you mention are of course different and some of those kinds of bugs aren't worth the trouble of even trying to test, but if your game is shaped in a way that it is basically a collection of code modifying data that represents the game, you can eliminate the need for a lot of manual QA testing by just automating it.