I first thought the name must be a clever pun when I saw that the original JPEG was dated 1992. But sadly my brain was off by a decade, JPEG XL is in fact not JPEG 40.

They could have gone with JPEG XXL or JPEG XXX but the former is a bit too fanciful and the latter might cause some adoption problems outside a certain niche industry.

JPEG MMXXI ?

JPEG-XL can encode huge images, whereas JPEG is limited to a maximum width and height of 65535 pixels.

Is support for 5 gigapixel images really such a killer feature to put it in the name?

That's not where the name comes from :)

https://en.wikipedia.org/wiki/JPEG_XL#Name

It's also very similar to JPEG-XR.

And JPEG XT. And JPEG XS.

I prefer calling it JXL (or jxl) which is visually sufficiently unique to avoid confusion.