I'd agree with that if we were talking about the first HW game, but AoC felt a lot like playing BotW with a different combat system. I think they did a far better job at incorporating the series' core elements into it, even exploration with how big and intricate the maps are and the hidden...
It being an AU doesnt make it non-canon, it just means it doesn't impact BotW's story.
Now, if As for now, though, the only real reason to think it's not canon is because it has "Hyrule Warriors" in the name. Which is fair of course.
I think Age of Calamity can be considered mainline. Ultimately, Koei did make the game, but they didn't make the story. Aonuma and Fujiabayashi conceived it, and Fujibayashi was the scenario supervisor for the whole game. I think it's in an awkward spot, where the only thing separating it from...