Leaving everything else aside because it's been hashed out a plenty by now... Sage absolutely doesn't understand what she is talking about.MC: You don't understand (She understands just fine you just don't care)
Sage: No, you're the one who doesn't understand. (Exactly)
For a very simple reason: Sage apparently doesn't have even a single close friend who was her actual friend for more than a couple of years, nevermind one she'd have spent half of her childhood/teenage years with. She is unable to put herself into Fuckface's shoes here because she utterly lacks the needed frame of reference.
So she really doesn't understand what Zoey is to MC and how he feels about her. Because she doesn't know first thing about this sort of relationship.