If you do not run that command before /boot fills up, and you have a full /boot with a partially installed kernel, then that command fails. So this works fine if you remember to call it regularly, but it does not solve the problem once it occurs.
Following the chain of links and answers and explanations, we come to the conf file that says in the comments that it commonly results in two (2) kernels being saved, but can sometimes results in three (3) being saved.
IOW, it does automatically remove old kernels, it just keeps the last 2-3.