I understand the bootloader can't mount the ext4 partition to pull the boot.txt/autoboot.txt out of it. In fact, consulting the wiki again it claims that partition must be either ext2 or FAT, but that that boot.txt can load a kernel from FAT, ext2, ext3 or even ext4 according to one bit of the text (though it doesn't document the syntax for loading a kernel from ext4) and from there the kernel can read most formats fine.