Why self=[super init] ensure the singleton pattern?

I read many implementation of singleton pattern in Objective-C, and lots of the code contain such a line in the init method:

if ((self = [super init])) {
   ...
}

And according to this page, which is referred by many questions regarding [super init]: https://www.cocoawithlove.com/2009/04/what-does-it-mean-when-you-assign-super.html

self = [super init] "always returns the singleton instead of any subsequent allocation".

But why? The following is my implementation of a shared singleton

#pragma mark -
#pragma mark Constructors

+ (VoiceProfileManager*)sharedManager {
    static VoiceProfileManager *sharedManager = nil;
    //Executes a block object once and only once for the lifetime of an application.
    static dispatch_once_t once;
    dispatch_once(&once, ^{
        sharedManager = [[VoiceProfileManager alloc] init];
    });
    return sharedManager;
}

- (id)init {
    if ((self = [super init])) {
    }
    return self;
}

@end