Yeah, that is a good point. It must be weighed whether these particular 0.3% are worth it against other potentially contentious issues (and we should additionally consider whether it is likely to become more in the future, or if it's simply a large enough number to matter over the years regardless), hence also my cagey phrasing of "I'm not arguing the Autobahn [in particular] must be high on the priority list".

Still, though, simply pointing at cruise ships and forgetting about potential gains here is not the most constructive thing to do in my opinion.

I am not sure how to put this nicely. Autobahn drivers that currently drive at the maximum speed are already quite likely already frustrated. They probably would like to fly a plane and they are constantly hindered by other cars on the road.

In other words, they are the kind of egoists that are unlikely to support any form of CO2 reduction already.