Your impression is correct; when JPEG XL was first introduced, it did just as well or better than AVIF in many cases. But in short, work on JPEG XL encoders slowed down, so they fell behind, and JPEG XL now loses to AVIF in some areas.

As the article points out, this might change yet again in the future, depending on how much AVIF keeps improving and whether JPEG XL development picks up steam again and how far it’ll go.