If you're showing the user something that they may mistake for complete but poor quality, that's a bad experience.
Some users have uncorrected astigmatism. In the general population, the average uncorrected astigmatism typically falls between 0.50 and 0.75 diopters (D), and ~15 % more than 1.0. The high astigmatism people will see images blurred in any case and don't benefit from the fine structural detail JPEG XL can give. People with glasses and the lucky ones with low astigmatism will see the details clearly.
Sometimes passionate ends of different quality of vision seem to have a discussion on what makes sense as the web quality, with one emphasizing quality 40 being more than enough and another requesting a quality of 95.
The same kind of individualism exist also in temporal cognition and perception.
And that comparison was made for a JPEG-XL focused site, and uses quite large images that are then scaled down dramatically (at least on mobile) for display which is probably not an ideal visual test anyway (but possibly necessary for there to be enough time to see any difference)
Personally I don't like progressive rendering at all, but it's interesting that this same guy claimed elsewhere that it's super important on a very bad connection to get an early hint of what an image is. When someone points out that the "the results speak for itself" cherry picked example is absurdly misleading, and in most cases JXL is vastly superior at that case, instantly their opinion changes such that it shouldn't look like a poor quality image but it's somehow also a win that AVIF takes longer to give you anything at all to look at.
Which is incredibly weird, given that AVIF also gives you a terrible quality image, it just takes longer to show it. So it sounds like they're completely against progressive rendering? Unless they think it gives AVIF a benefit?
But it's an amazing demonstration of someone with an agenda. Utterly bizarre.
I'm not against progressive rendering - I fought for it to be part of the jxl-rs integration in browsers. I've written articles about it https://jakearchibald.com/2025/present-and-future-of-progres....
Just because someone has a different opinion to you doesn't make them part of some grand conspiracy.