Yes, it is an unitended consequence of the fact that argparse types are arbitrary single argument functions (that take an arbitrary string as the argument and convert it), and bool is a single argument function.  Unfortunately we're stuck with it now.  The correct answer, of course, is to write your own bool type converter if you need one.

It is possible there should be a documentation mention that bool is not approprate as a type function, since int and float, for example, are explicitly mentioned.  If you want to submit a patch to that end I'll reopen the issue.