Is the original intent of strncpy() germane to the GPs comment? Explicitly stating the max length to copy is an effective tool for avoiding buffer overruns, regardless of whether the designers imagined that important use case.
If you look at this function assuming it's a safety feature, that's a huge surprise, and indeed if you were skimming you might miss what it does because (in the context of "it's a safety feature") this is an insane choice. "Why would you do that?". Well, because it's not a safety feature.
The perf cost isn't what you'd expect from a safety feature either. Suppose we have a 1MB buffer, and we strncpy "THIS" into it using n = 1024. That's just four bytes right? Nope. strncpy() will write "THIS" and then 1020 zero bytes.
Not only that, but (because of its actual purpose) it also fills the buffer with nuls, which is a complete waste of resources.
So yes, the original intent of strncpy() germane to the GPs comment, because it makes strncpy actively dangerous and complete shit when working with C strings.
Calling a bespoke byte-sequence data structure a "string" is inaccurate. Treating strncpy() as a string function is erroneous and can easily lead to memory corruption.