Yep. It's just a mistake. A dumb one too, since it was so easy to test. :-(
Chalk it up to wishful thinking on my part. Or maybe just too much blind faith in the compiler: "It's something that would be easy to test at compile-time, ergo it must be tested at compile-time".
Still, at least it's not the worst mistake I made in PBP. ;-)
PS: I stand by the overall recommendation though. Error messages that point users to the right place are definitely worth the (tiny) overhead of passing named args in a hash.