The reason is C had no built-in boolean type, so boolean values use an integer type just like error codes. In C, zero means false and non-zero means true, so that’s how boolean values are represented, and hence a boolean value is_success is non-zero on success and zero on failure. For error codes, on the other hand, the convention is that zero means “no error” and non-zero means some specific error. So exactly the opposite. Unix also reflects this same discrepancy, in that a process (i.e. main() or exit() in C) returning zero means success, but in Unix C programs that’s boolean false. (Not to mention that for pointer return values, null often means failure.)

A valid criticism might be that one shouldn’t use true to mean success.