That page correctly describes the different entities that are all called ‘Canada’. It’s not wrong. The one that has its own monarch - the King/Queen of Canada -

That page correctly describes the different entities that are all called ‘Canada’. It’s not wrong. The one that has its own monarch - the King/Queen of Canada - is the modern one, established in 1984. Previous to that, the UK’s monarch had dominion over Canada. In 1984 the roles were made distinct.