Child has it. The way this study is written, we're asked to compare more or less the carbon impact of an empty gas tank and the battery pack. That seems a bit unrealistic to me. yes the drivetrain is different and therefore more costly, but when it comes down to it, looking at the examples in the paper, the comparison is between the battery pack and an empty gas tank. That seems disingenuous to the broader point of carbon emissions for the entire life-cycle of the vehicles, which presumably would include use.

Which approach is "more realistic" depends on whether you are evaluating the decision between an electric car and a gas car at the margin, or not.

If it's you as a consumer buying the next car to be produced, then it is entirely appropriate to compare it that way, because your single purchase will not change the entire life cycle.

I think their point was that it seems unfair to count up all the of the embodied energy of an electric car's batteries, but to only count the potential energy of the ICE car's fuel.