I think he's just using the dark knight as shorthand for something that shines the characters in a different light. Comparing dark knight to the tv show of batman is night and day
I don't think he realizes what a big mistake he made. Batman has had nuanced and intelligent (and yes, "grim") source material for eons now, and had a very dark tone to begin with (Bob Kane era). Power Rangers is a childhood favorite of all real nikkas born 85-90'ish, but let's be honest the writing is retarded, and the concept is ridiculous. I love MMPR, but as a serious action drama it's beyond salvage.
I'm hoping for the best, but this is at least 85% likely to be another bad Hollywood adaptation.