Who tf said lunas ending is the true ending?
The original series ended with Lune's ending, so that's my reasoning on why I believe Lune's ending in the "true" ending.
In the beginning, the author didn't have any plans to do a multiverse as he originally planned on only having Lune get an ending. However, as the series progressed, the author grew conflicted about having Yoshitake get the ending, which if she does, Lune's ending will be abandoned. After countless debates between him and his editors, the editor-in-chief told the author he can do both endings which latter spun off into having the last volume be nothing but alternative endings.
This is all explained at the end of volume 10. With that said, I don't think the author came out and crowned lune's ending as the true ending so it's up for the readers to see which ending they think is the best.