I think Tegmark's claim is unequivocally that we should endorse Dark Artsy subsets of scientific knowledge to promote science and whatever needs promotion (rationality perhaps). So yes, the thing being claimed is the thing you are emotionally inclined to fear/dislike. By him and by me.

Though just to be 100% sure, I'd like to have a brief description of your meaning of "dark arts" to avoid the double transparency fallacy.