We don't even know if Socrates not writing his teachings was a principled stance to begin with, so "Socrates was right" is a shaky place to start your argument. Also "Socrates said writing is bad, so don't write in this piece of writing that you didn't force me to buy" is one of the weirdest pieces of circular logic I've ever come across.

Even if we take for granted that it was principled, it almost certainly wasn't pointed at marginalia. The argument (that Plato put in his mouth, through written text, no less) is that writing that substitutes for internal recollection can't answer back. It's an argument against outsourcing a thought to fixed text. A margin note is the opposite: it's the reader talking back to the text.

You have a personal preference about buying unmarked books, and you retreat to a Grand Unifying Socratic Theory whenever someone challenges it.

> it almost certainly harms their own understanding of the book

What? For all people who ever mark up a book in the margins or highlight passages? How could you possibly know that?

> they're doing something that is guaranteed to harm overall human knowledge

Also what? Human knowledge is necessarily a collaborative effort. Reading a text with another person's notes alongside it is just another way to engage with it. I'll maybe grant you highlighting since it's been shown to not help with retention all that much.

We have a deeper understanding of some of the most famous thinkers in history because we have books that they marked up to help us understand how they engaged with specific texts. This is invaluable for "overall human knowledge."

Herman Melville marked up his copies of Shakespeare's works and that shaped how scholars read Moby-Dick. Did he "harm overall human knowledge" by doing that?