I don't think we're even debating.

"I can teach you this only if you already know it" seems like bad teaching to me.

Yes, that is the definition of bad teaching. My assertion is that CS departments have gotten so damn complacent about receiving a steady stream of autodidact programmers as their undergrad entrants that they've stopped bothering with actually teaching low-level courses. They assign work, they expect to receive finished work, they grade the finished work, but it all relies on the clandestine assumption that the "good students" could already do the work when they entered the classroom.