to launch illegitimate vc title you need to launch a custom firmware first (haxchi or mocha) or it wont work because the non cfw wiiU will know its illegitimate
so imagine if you want to make something illegitimate autoboot, the console will say "no" and thats why it will be a brick
and thats why you need a legitimate vc title, haxchi will replace it, like its wearing its identity, its software skin, and it will be authorized to be launched without cfw (when you will lanch haxchi on a legitimate title the non-cfw wiiU will say "hi oh its brain age training, your papers seem to be in order sir, have a good play sir" and haxchi will launch instead, and if you autoboot on it with cbhc it will be the same
tldr : you need a legitimate title because if its not your console just wont be able to boot it