Your example, though, outright assumes the claim is false because you have different error rates for different languages.
I agree the metric is coarse, but I think it's a decent stand-in for things that matter. More lines of code tend to mean more concepts unrelated to the problem you're solving, more assumptions and a greater cognitive load. All of these introduce more opportunities for error. Consider the problem of concatenating a sequence of text, and putting commas inbetween each distinct entry. In Python:
concat = ','.join(texts)
In C++, this would be
string concat;
for (list<string>::const_iterator i = texts.begin(); i < texts.begin(); ++i) {
concat += *i;
}
There are more opportunities for me to make mistakes in the C++ code. I've introduced more concepts (using iterators to access a sequence, using iterators to access a value, building a string) which increase my cognitive load. Hence, I'm more likely to have more bugs in this code, despite solving the same problem.
I don't even want to write the C code to do this right now, because I would have to deal with the following concepts: allocating sufficient memory for the C-style strings and ensuring I'm not accidentally overflowing that memory; determining what kind of sequence texts is (native array or my own linked list?); determining how to iterate over that sequence, and how to "add" strings.