There can be multiple reasons for heavy cars to be bad. Additionally, extra heavy cars wear roads much faster.

It's nice to have a way of recuperating energy but it doesn't solve everything.

They didn't say it solved everything

Actually if I follow the conversation, the first person said “storage density matters”. The second person said “actually it does not because the energy gets recovered”. The follow up said “actually there are other reasons why weight is a problem.”

So the first person never set goal posts around efficiency, that was an assumption by the second person. The first person could always have meant that the collision risks from heavier vehicles are higher, while the second person misunderstood their meaning. No moving of goalposts here.