There's also another viewpoint. POSIX defines a "line" as something that ends with a newline (see here, found via this StackOverflow question), and I assume that that definition is related to, if not based on the Unix convention of always ending text files with a newline. Under that definition, counting newlines is also counting lines.
Thanks for replies. On one side we have 'wc' and POSIX, on the other BufReader.
Since posting my question I discovered rust corutils which conforms to wc and suggests that BufReader's different notion of a line is not regarded as a bug.