Just because I am the type of person that I am I have to point out as to whether an e-bike weighs more than a regular bike depends highly on the regular bike and also the e-bike honestly. They made some heavy ass acoustic ones, they make some lightweight e-bikes. Latter with little range, mostly, to be fair. I say this to ask if somebody shipped some heavy ass steel bikes, would you be concerned as per CO2 Output?

I know sea freight is very efficient, but there's no reason not to account for the difference if we're making the comparison?

I mean it's there, but it's genuinely just marginal. Like 0,03% of the CO2 Budget of any given bicycle were it ridden.

Now the resource extraction I don't have numbers on and it probably sucks. People buying E-Bikes to ride them 200km over a 10 year period probably also isn't great for CO2-output - in the context of bicycles - but if we assume what gets shipped gets used it doesn't matter

There's a couple of issues at play which mean it doesn't work exactly like that.

Firstly larger ocean freight don't scale proportionately to weight linearly. So even if we assume that ebikes would mean a full doubling of the weight of a given trip it wouldn't require double the energy and therefore emissions to do the trip. It will depend on the exact vessel but an estimate from here is for each additional 100 ton of mass to a container ship it would use an additional 0.0714 of a gallon of fuel. Its very cool physics which is largely just down to the sea doing most of the work carrying the weight itself (the same works for different reasons for rail but all other modes have much closer to linear scaling).

The other factor is that in practice the energy and emissions are the result of whole systems and trips are not always operating at ideal conditions. So its quite hard to judge what actually changes in a while system if there's an increase in some weight of some products.

These are the reasons that additional weight in ebikes doesn't come out to a huge increase in shipping emissions when its all worked through.

edit: paper uses imperial ton not tonne - corrected