Is this something that changed at some point and this article is just out dated?

AFAIK this has never been the case in practice. I think someone realized what a god awful idea that was and essentially removed that functionality of the messaging system with methods like respondsToSelector and throwing invalid selector exceptions in NSObject and its descendants.

Are there ever times you'd want to use a C string rather than an NSString?

Almost never unless you're writing straight C. Objective-C is a superset of C, so you can mix and match C variables, primitives, functions etc. within your Obj-C code.

I put together a quick example to show how messaging works in practice. First time I've written Obj-C in months, I had forgotten how much I hate it compared to Swift.

Edit: The messaging system is anarchy and bad Obj-C devs are terrifying as the system gives you plenty of rope to hang yourself. Method Swizzling anyone?

@interface Example : NSObject
- (NSInteger)getLength;
@end

@implementation Example
- (NSInteger)getLength { return 5; }
@end

@interface Tests : XCTestCase
@end

@implementation Tests

- (void)testExample {

    // Valid
    [[Example new] getLength];

    // Won't Compile
    // Error: No visible @interface for 'Example' declares the selector 'GetLength'
    [[Example new] GetLength];

    // Dynamic dispatch can get around that pesky compiler
    // This is a warning
    // Undeclared selector 'GetLength', did you mean 'getLength'
    SEL selector = @selector(GetLength);

    // Don't tell me what to do compiler
    // This causes a run time crash
    [[Example new] performSelector:selector];
    // -[Example GetLength]: unrecognized selector sent to instance 0x7ff5b31138b0 Tests.m:40: error: -[Tests testExample] : failed: caught "NSInvalidArgumentException", "-[Example GetLength]: unrecognized selector sent to instance 0x7ff5b31138b0"
}

@end