I interpreted the comic as meaning not any old tar command line, but a command line for a specific intended purpose. I had a job interview once with a test consisting of a whole list of 20 unix questions, one of which was "you need to do x, y and z in tar, what's the command line?". I got about 5 of them, but spent the 5 minutes critiquing the test and suggesting ways to improve it. I got the job. Apparently my 5 answers were the best anybody got anyway.

That's interesting, I always interpreted it as "enter any valid tar command".

That's the most obvious interpretation, but it doesn't really make much sense. At the end of the day, I suppose it's just a joke that tar command line arguments are bizarre and arcane.