Since we're speculating without proof, my guess is the (not biblical) saying has always meant exactly what you think it means, and someone invented the "blood =

Since we're speculating without proof, my guess is the (not biblical) saying has always meant exactly what you think it means, and someone invented the "blood = covenant, water = familial ties" inversion just to be edgy, ironic, for effect (whatever time period that was). And as social media has taught us, humour and irony get lost very easily. I have to wonder how much of history we misunderstand because we didn't understand the tone.