My impression is it wasn’t so much about thinking they had a unique idea that the others were “stealing”, as fearing the other books with similar themes releasing at a similar time would pull attention away from their book. Which is actually a bit silly, because generally the first thing someone does after loving a book with a particular genre/theme combo is go and look for more of the same. Cooperating with those other authors, so they all plug each others books (“Hey, if you loved my book, go check out X, Y, and Z too!”), would have been so much more productive.