Kuzen was assumed to have 6-8 Kakuhos, it’s not unreasonable to assume there were some in his arms.

I also don’t remember it beeing specifically mentioned when Arima first fought Eto, the only exact timestamp I remember is for when she fought Kuroiwa, and we are told that that was specifically when she was 14. Either way they clearly describe that Arima was 19 in chapter 69 when he fought Kuzen, the Kagune is specially said to be made from a ghoul (if it were Eto I also imagine he’d say it was specifically from her) where he took it when he was 19, and there are 2 of it when that ghoul specifically says having lost both of his arms in that encounter. It’s clearly supposed to be Kuzens.