so why do you need another kernel with lockdown disabled by default to be shipped?

If you have the patch that sets default lockdown policy based on secure boot state, you don't. If you don't have that patch, you do.

Why would an attacker not force that kernel to be booted when he gets control over the machine?

Because it wouldn't be signed.

Having a command line parameter to disable lockdown would make it obvious both in the journald logs or in /proc/cmdline.

Both of which could be compromised after an attacker adds the option and livepatches the kernel.