I'm pretty sure the Chrome team knew at the time they dropped JPEG XL that there was a decent chance Apple would implement it--there's certainly enough of a backchannel between the browser teams at Google, Apple, Mozilla and Microsoft.
But if the Chrome folks did know they had all three major browser engines were ready to go, along with the positive noises from Facebook, Adobe, other parts of Google, and a few other notable folks, that seems like a solid base of support, unless you're really expecting the world to go all-in on a format before a browser supports it without a flag. So I guess I wonder if it's that the Chrome team had set the bar high, that they hadn't expected Apple to support JXL at the time they made the call, or something else.
I think it’s something else.
I’ve read enough CSS Working Group (and other groups) meeting minutes to know when Google wants to implement something, that’s what they do, often when the WebKit and Mozilla teams don’t agree.
Whatever happened for Google to reverse their position on JPEG XL, I don’t believe it has anything to do with the technical merits of the format or whether Apple had planned to implement it.
Just the JPEG recompression is arguably worth the price of entry; it's less savings than AVIF, of course, but it's easier to adopt now, where re-encoding everything as AVIF is a much larger hump to get over. Its other modes provide some real benefits for users for very-high-quality and lossless images as well as any situation where progressive display helps. For folks producing images, is much easier to fit in than AVIF for anyone who needs to encode a lot on the CPU.