> In that loop one should check that len is at least 1 first...
No. The code is correct in that regard if you use int for i and len. Having to check len is an artifact of using unsigned.
> I read that page, or at least some of it, it mostly seemed to be opinion rather than solid reason.
I'm not talking about the comments. I'm talking about what Bjarne Stroustrup (the designer of C++) and Chandler Carruth (Google's LLVM lead) are saying in the video (and which is summarized in the top comment).
Only if a sub 1 len is valid input, which it may not be. Either way the example is contrived and does not cover all other situations in which a scalar quantity is required. Not all are loop controllers.
I did not watch the video, no time right now. The top comment tells us what they think, not why.
I read that page, or at least some of it, it mostly seemed to be opinion rather than solid reason.