There is no need for a magnum primer with 231 in a .357, IMHO - or at least I have never seen or heard of a need. I wonder if they didn’t specify it just because they used the same primer for everything else in their .357 data? I have been using 231 with Federal 100 primers in moderate .357 loads for years, if not decades. Never saw any indication that made me think a magnum primer might offer an improvement.