Tests are great. No harm in tests.

I just don't think that we have sufficient evidence to conclude that using type systems to aid the development of concurrent code actually leads to better concurrent code. Therefore, I err on the side of not using the type system for that purpose so that I can use it for many other things that I think it's really good at.

(Just take a look at JSC's source code if you want to see our extensive reliance on those areas of C++'s type system that we can use to get hard guarantees. It's not that we don't like types. It's that we use types in an evidence-based way in those places where they actually help us catch bugs.)