> The original comment has the obvious implicit judgment that a greater CO2 footprint is a bad thing.

No it did not contain any judgement. That is your reading and not the comment.

> This is shared context.

The world is a large place and different people share different context.

> The purpose of the comment is clearly to shame the person for having such a high carbon footprint, otherwise there's no purpose in bringing it up.

Informing perhaps?

I wonder what your gripe is: Do you think it's inappropriate to mention the CO2 at all? Or do you think that the poster should have said it in a different way? In what way?