Lydia Davis points out that the French cognate of "superstitious" can mean: “excess of exactitude, obsessive preoccupation." I think that this what Borges means when he says an excessive investment in the style of writing is a superstition. I mean, literally he doesn't mean something like a fear or black cats or the number 13. I can see the connection between the two meanings: irrational investment in something that really makes no difference. The faux amis are countless.
Of course, there is an irony in that Borges is himself an unsurpassed stylist.