Good catch! We were moving quickly trying to get this blog post out before the standard was implemented. We've updated this and it's now a smaller file size.
The .avif file in the example was accidentally saved as larger than the JPEG file which makes it a poor use case. We compressed the .avif file with the correct compression quantizer with avifenc to higlight the difference.
The JPEG file is 112kb and the .avif file is now 55kb.