There's no point in saying technically, he is the King of Canada. That being said the Crown is mostly symbolic like the UK, however symbols can be very powerful. By inviting the King to open Parliament, Carney is symbolically reminding Trump that Canada is much bigger than just one country. Likewise the Chiefs are reminding Canada, specifically Alberta, that the Crown holds the treaties, not Canada.
Basically this is like getting picked on in school then bringing your big brother to lunch. He doesn't have to do anything, the point is to remind your bullies you have some pretty strong support, and they should think again about continuing to bully you.