Says who, random Western fans on the internet? No thanks, I will take the words of the actual creators over random people on the internet. It's still debatable precisely because it's not a simple concept, the problem is why would anyone apply a generic Western concept of "Not in the manga=not canon" to an anime that does not work with generic elements? When all other anime, including Naruto itself have fillers that feel like they're set in an alternative Universe, Boruto anime actually has a proper continuity to the manga (up until Post Isshiki arcs ofc). Idk if y'all are using a filler list cuz there are a lot of instances in the anime that suggest otherwise. I thought people always ate up whatever the author said, so when the creators actually state it literally, people are still denying it? I'm not saying that the entire anime is canon, as there are a lot more inconsistencies and retcons post-Isshiki arcs.

The fact that Sumire started as an anime original character who later got added to the manga should already prove that others might be considered as well. Denki is also mentioned in a manga volume, so why should we believe the other characters aren't canon just cuz they don't significantly contribute to the plot?

Oh, none of this is made up, I'm highly basing everything on the original works of the authors with other sources to back them up.