It's not that metal can't scratch it; it's that NON-stick surfaces are highly vulnerable to being scratched by metal, whereas cast iron is much, much less so, and scratches can heal as long as you cook with more oil.

I think what was meant (and that I can anecdotally confirm) is that metal utensils do a much better job of stopping what I will call "gunk film overgrowth" during cooking.

Iron is a harder metal than 'steel'.

Not true.

Steel is about 5.5 - 6.3 and Iron is around 4.5 on Moh's Hardness Scale. Higher numbers are harder.

They do scratch it, that's why they keep it smooth, but the iron is not a coating and is non-toxic, so it doesn't matter about wear.