Does `copy` really strip off mutability?

mute.m

#import <Foundation/Foundation.h>

// clang -g -framework Foundation -o mute mute.m

int main(void) {
    NSMutableString *blarg = NSMutableString.new;
    NSString *copy = blarg.copy;
    NSLog(@"%@ | %@", blarg.class, copy.class);

    return 0;
}


#if 0

% clang -g -framework Foundation -o mute mute.m
% ./mute
2018-12-18 10:30:27.461 mute[84191:14368442] __NSCFString | __NSCFConstantString

#endif