The point is that you claim to have "the" (best?) way to do proper unit tests, but it still fails in basic cases like some combination of paths through if state

The point is that you claim to have "the" (best?) way to do proper unit tests, but it still fails in basic cases like some combination of paths through if statements being buggy (even if you properly covered each one in isolation). You normally cannot prove general correctness with unit tests. You can try to probe interesting points in the input space, and different coverage metrics encourage different levels of rigour and effort with this, leading to different fractions of bugs caught, but you'll never have a guarantee to catch all bugs (short of formal proof or exhaustive input coverage).