I do not believe MS ever made a claim that all first party games will be native 4K. It would be good if you can find that bit of info but I did some searching and could fine nothing.
They didn’t. Per usual, people are on some bullshit. Shannon Loftis did say Microsoft’s own titles launching during the Scorpio window would be native 4k. Which afaik they delivered on. Someone can prove me wrong though.