Welfare's giving sell-in estimates. Those don't even contradict Ampere so I'm not sure why you're doing this credibility ranking?Welfare can have a methodology on his own, but it is not close to a access NPD, GSD and Media Create data in any single way. Of course i will take their data with more credibility.
If we take your credibility argument to it's logical conclusion though, Nintendo has greater access to data and resources than Ampere, thus they must also be more credible. And since Ampere was credibly too low on Switch then, who's to say they can't be on Xbox? That's the thing about estimates...