The other person was just wrong.

Large scale Hydrogen generation isn't generated in a fossil free way, Hydrogen can be generated is a green way but the infrastructure isn't there to support SLS.

Hydrogen is high ISP (miles per gallon) by rubbish thrust (engine torque).

This means SLS only works with Solid Rocket Boosters, these are highly toxic and release green house contributing material into the upper atmosphere. I suspect you would find Falcon 9/Starship are less polluting as a result.

Lastly the person implies SLS could be fueled by space sources (e.g. the moon).

SLS is a 2.5 stage rocket, the boosters are ditched in Earths Atmosphere and the first stage ditched at the edge of space. The current second stage doesn't quite make low earth orbit.

So someone would have to mine materials on the moon and ship them back. This would be far more expensive than producing hydrogen on Earth.

Hydrogen on the moon makes sense if your in lunar orbit, not from Earth.