I explicitly wrote "or whatever test" as a comment to the code snippet, and obviously the test was not the point of my comment.
Anyway I think most programming languages nowadays have their source encoding specified as UTF-8 or at least something ASCII-like, so ('A' <= c && c <= 'Z') is in fact what I would likely write, and using isalpha() would technically be a bug just as well.
My response was to OPs moving the goal post to "portably parsing ASCII" in response to his suggested replacement for a C library function not being portable on non-ASCII systems, which make no sense.