Mining

Yeah, it will certainly release the same amount of carbon dioxide while being burned as regular diesel -- no getting around that.

But the idea is that you extract the carbon dioxide from the air (which costs electricity), then split it into carbon and oxygen (which costs electricity), then also split some water (which costs electricity), then combine the carbon and hydrogen to make fuel (which also costs electricity). The efficiency of the whole process is terrible, but if electricity is so cheap that it is almost free, then you can start to entertain the idea.

There's a lot of assumptions there about electricity being super cheap, and nuclear being so abundant. Nuclear plants aren't free to build or operate either.

That said, this same technology can make methane, and methane powered rockets are likely the future. So for a limited consumer of fossil fuels (like rockets), maybe it makes sense. For things that cannot be made to run on batteries, regardless of how efficient the batteries become.