Why do they add yet another image format after AVIF and JPEG XL?

This is premature, Webp2 is still under heavy development, no release has been published and the bitstream is far from being defined. Also we don't yet how the market will receive AVIF and JPEG XL, making investment in yet another format quite risky.

IMHO (when using the comparisons linked without description), WebP actually does a visibly better job than WebP2 at the moment for all sizes, anyway (better retention of small detail and practically same file size, even though WebP2 tends to preserve more vibrancy at very low bitrates) so it's definitely not even interesting as yet another image format.
I'd downvote this if that was possible.

I agree that work on this should NOT be started in any way.
JPEG XL for example already shows from small to massive advantages over most formats, and they are far ahead in implementation - they and AVIF should be finished first and see how the market reacts.

WebP 2 will not be released as an image format but is used as a playground for image compression experiments.

(https://chromium.googlesource.com/codecs/libwebp2/+/1251ca748c17278961c0d0059b744595b35a4943%5E%21/)

So this bug can probably be closed?

Based on it just being an experimental playground. Feel free to reopen if that changes.