It's an absolutely ridiculous point, and the guy you're responding to (who has commented almost 40 times in this discussion) has a hilariously slithery opinion that somehow always finds a way to hold every finding against JXL.

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.