Cus the anime doesn't just follow the manga, it doesn't try to catch up with the manga it works in a separate plane so it's canon cus it doesn't interfeare and try to come up with things just to pass the time.
Who cares? The anime is better anyways. Like I can't even laugh enough at the idea that one media form is better or more valid than the other simply because of one word. The anime has better pacing, better plots, better art, and better handling of the characters.
Yes this is right, the story of the manga and anime are drafted by the same person in the first place and, unlike Naruto's filler, the extra content included in Boruto enhances the overall narrative, including relevant information that has a real impact on the storyline, like Boruto getting his first summoning which was also a huge plot point in Naruto. Boruto anime is definitely the canon.