You can use kexec to load a different Linux kernel from a Linux kernel.

Probably slower and perhaps less compatible than using GRUB though.

That’s the neat part - you install GRUB if that’s something you care about. For the 98+% who will always use the newest kernel, and can tell the system to (hypothetically) use a different kernel on future reboots after the system has loaded, it won’t be an issue.

They address this use case in the video. Their loader can show a menu.

Via EFI probably.

Does that mean the UI you will use to choose the kernel will probably be the bios?

Yes. If your UEFI doesn't have a good enough interface for selecting entries or temporarily modifying a kernel bootline, you can still use a bootloader, but a minimal one like systemd-boot instead of GRUB. All it does is show the text menu and then execute the UEFI binary for that entry, which in this case is the kernel's UKI binary, so all the heavy-lifting of LUKS, filesystem drivers, password entry, etc is done by the kernel and there's no complexity or duplication in the bootloader.