It could be that Linus, the people he chose to be his "captains", the design and code review process, and the sheer amount of linux usage anywhere maybe enough.

It could be that Linus, the people he chose to be his "captains", the design and code review process, and the sheer amount of linux usage anywhere maybe enough. The linux kernel might be the most analyzed (by people and machines) piece of software there was. But then a lot of business-logic software is out there just to support client-A able to do rpc calls to your service, and usually one or two engineers implement it, few more look at it, gets deployed, and it's expected to run flawlessly for long time. Then someone else modifies it later, and the original "developers" are gone (another project, company, retired, etc.)... Definitely not the case with Linux. So my take is that such software needs TDD to hold on it's own, such that it could be developed further by complete strangers.