the title text says 'Loaded by the EFI stub on UEFI, and packed into a unified kernel image (UKI), the kernel, initramfs, and kernel command line, contain everything they need to reach the final boot target' which sounds like they're not talking about using two separate kernels, one for the bootloader and one for the final boot target, but rather only one single kernel. possibly that is not the case because the actual information is hidden in a video i haven't watched
https://news.ycombinator.com/item?id=40909165 seems to confirm that they are indeed not saying what you thought
edit: they're proposing both configurations