> its simply the fact they are a person living in the world.

That's the whole point! If a task requires some time from a human, then you have to include the appropriate fraction of the (huge!) CO2 cost of "being a human" - the heating/cooling of their house, the land that was cleared for their lawn, and the jet fuel they burn to get to their overseas trip, etc, because all of those are unalienable parts of having a human to do some job.

If the same task is done by a machine, then the fraction of the fixed costs of manufacturing the machine and the marginal costs of running (and cooling) it are all there is.