How do I clean my cast iron grill pan?
I own a similar square grill pan and have always had luck cleaning it with a stainless steel scouring sponge and lightly soapy water.  The spongy texture is very effective at getting down into the grooves.  No need to be especially vigorous - a couple quick passes will take out accumulated residues, and you can use a regular sponge afterward if you like.  I've no idea how they season these, but it seems to hold up quite well over time.  As near as I can tell, you can also re-season these just like any other cast iron when it does degrade.

Their statements about enamel are probably referring to the outer, colored finish - I also own a Le Creuset dutch oven which has the same enameling inside and out.  Care and feeding of that surface is of course totally different from cast iron.