He's saying that a strongly typed language with unit tests is not enough to prevent bugs. And he's right.

Strongly typed languages, and unit testing, do prevent or detect many kinds of bugs less expensively than the alternatives. That is why they're worth using.

Similarly, not all diseases are prevented by vaccination, but that's not considered a good argument to stop vaccinating people against those diseases that are prevented by vaccination.

I agree with you, but you're being argumentative. No one here is saying that we should stop unit tests because they can't find all the bugs.

Instead, the focus should be on what else we can do to improve the bug detection rate. This is an area that needs further research (and I would start with the observation that the quality of the unit test varies dramatically depending on who writes them).

The debate on dynamic vs strong is more important than testing vs no testing.

Why? Explain yourself.

Yes, this was the point of his quote. Strange how this point was missed in the other comments. Of course, Hickey is aware that not all languages are strongly typed.

There might still be far fewer bugs.

There's always at least one type checker, the one running in your head. Some languages provide another type checker to help offload some of this cognitive load.