It's been a while, but I believe this video was where I heard it. From memory (I'm out right now and can't rewatch to verify) it was specifically the per-kilometre carbon emissions, not taking into account manufacturing costs.

Obviously there's some fuziness depending on your diet and the power source used for charging. A vegan who would be charging in a coal-powered grid is going to look better, relatively speaking, for an analogue bike than someone who eats multiple kilos of red meat every week who has solar panels.