The translation given is “Since you do not wish to obey me, I will follow you.”
σὺν means “beside, with”. So doesn’t the last part mean “I will follow with you.”? I don’t get this usage of σὺν here.
I’ve just come across the translation - it’s on p. 340 (section 29.4) of the Cambridge grammar. The quotation is used to demonstrate the contrastive emphasis of the personal pronouns ῡ̔μεῖς and ἐγὼ, which are underlined. That could be why they overlooked the meaning of the preposition. It’s not mentioned in the list of corrections on Every van Emde Boas’ academia page - I don’t know if there’s an updated version anywhere.